Fix sessioninfos race that can cause crash during USER_CHANGES handling
Fix sessioninfos race that can cause crash during USER_CHANGES handling

When stress testing etherpad-lite we occasionally got this error:

TypeError: Cannot read property 'author' of undefined
    at /home/etherpad/etherpad-lite/src/node/handler/PadMessageHandler.js:556:47

handleUserChanges was accessing sessioninfos[client.id].author in a callback,
after spending some time in the loop that updates the changeset to the
latest revision. It's possible for a disconnect request to be processed
during that loop so the session might no longer be there.

This patch fixes it by looking up the author at the start of the function.

marcstreeter/etherpad-lite,2-B/etherpad-lite,aptivate/etherpad-lite,mcanthony/etherpad-lite,jeredkenna/pad,syci/etherpad-lite,liuliming2008/smock-server,delkyd/etherpad-lite,skulbrane/etherpad-lite,tiblu/etherpad-lite,tiandishi/wk,2-B/etherpad-lite,neynah/etherpad-lite,tiblu/etherpad-lite,cloudfoundry-community/etherpad-lite-cf,kentonv/etherpad-lite,cjb/etherpad-lite,Savi1234/savisql,storytouch/etherpad-lite,cmbirk/etherpad-lite,morpheyesh/etherpad-lite,skulbrane/etherpad-lite,Savi1234/savisql,adrienstoffel/ctfpad,Stackato-Apps/etherpad-lite,wonderxboy/etherpad-lite,fourplusone/etherpad-lite,wonderxboy/etherpad-lite,webzwo0i/etherpad-lite,University-of-Potsdam-MM/etherpad-lite,PuZZleDucK/etherpad-lite,sudowork/interview,emilyxxie/etherpad-lite,asac/etherpad-lite,cjb/etherpad-lite,lpagliari/etherpad-lite,goldquest/etherpad-lite,ashwini-angular/etherpad-lite,thebestwaytodesignbuildandshipsoftware/etherpad-lite,ether/etherpad-lite,wonderxboy/etherpad-lite,calangohc/etherpad-lite,coreyaus/etherpad-new,viaregio/etherpad-lite,adrienstoffel/ctfpad,girgen/etherpad-lite,fread/etherpad-lite,joequant/etherpad-lite,0x46616c6b/etherpad-lite,storytouch/etherpad-lite,0x46616c6b/etherpad-lite,pymjer/etherpad-lite,thomasrussellmurphy/etherpad-lite,meticulo3366/etherpad-lite,delkyd/etherpad-lite,joequant/etherpad-lite,cloudfoundry-community/etherpad-lite-cf,goldquest/etherpad-lite,ujdhesa/etherpad-lite,coreyaus/etherpad-new,adrienstoffel/ctfpad,nka11/etherpad-lite,ocdtrekkie/etherpad-lite,ocdtrekkie/etherpad-lite,asac/etherpad-lite,tudorilisoi/etherpad-lite,calangohc/etherpad-lite,Gared/etherpad-lite,ktdev/etherpad-lite,xavidotron/etherpad-lite,mani89/etherpad_ncloud,KenanSulayman/etherpad-lite,kenonelah/FactsheetPlugin,thomasrussellmurphy/etherpad-lite,bainsy88/etherpad-lite,viaregio/etherpad-lite,eXcomm/etherpad-lite,liuliming2008/smock-server,Atober/etherpad-lite,fread/etherpad-lite,mcanthony/etherpad-lite,ZackC/MirrorTest,eXcomm/etherpad-lite,bjonnh/etherpad-lite,bjonnh/etherpad-lite,aug-riedinger/etherpad,asac/etherpad-lite,Stackato-Apps/etherpad-lite,indykish/etherpad-lite,kenonelah/FactsheetPlugin,marcstreeter/etherpad-lite,marcstreeter/etherpad-lite,xavidotron/etherpad-lite,Gared/etherpad-lite,2-B/etherpad-lite,thomasalrin/etherpad-lite,cdg46/etherpad-lite,kentonv/etherpad-lite,2-B/etherpad-lite,yijunwu/etherpad-lite,cmbirk/etherpad-lite,yijunwu/etherpad-lite,cjb/etherpad-lite,tudorilisoi/etherpad-lite,thebestwaytodesignbuildandshipsoftware/etherpad-lite,bainsy88/etherpad-lite,lpagliari/etherpad-lite,goldquest/etherpad-lite,leangjia/etherpad-lite,skulbrane/etherpad-lite,unnamed89/etherpad,liuliming2008/smock-server,indykish/etherpad-lite,ZackC/MirrorTest,jeredkenna/pad,emilyxxie/etherpad-lite,ashwini-angular/etherpad-lite,DaveAtGit/secpad,KenanSulayman/etherpad-lite,cloudfoundry-community/etherpad-lite-cf,thomasrussellmurphy/etherpad-lite,rschiang/etherpad-lite,ErisDS/etherpad-lite,neynah/etherpad-lite,jeredkenna/pad,ZackC/MirrorTest,yijunwu/etherpad-lite,liuliming2008/smock-server,leangjia/etherpad-lite,fourplusone/etherpad-lite,jumperchen/etherpad-lite,aptivate/etherpad-lite,joequant/etherpad-lite,ujdhesa/etherpad-lite,eXcomm/etherpad-lite,vpistis/etherpad-lite,ZackC/MirrorTest,jeredkenna/pad,tiblu/etherpad-lite,emilyxxie/etherpad-lite,sudowork/interview,leangjia/etherpad-lite,rschiang/etherpad-lite,jumperchen/etherpad-lite,jumperchen/etherpad-lite,cmbirk/etherpad-lite,University-of-Potsdam-MM/etherpad-lite,morpheyesh/etherpad-lite,ujdhesa/etherpad-lite,pymjer/etherpad-lite,unnamed89/etherpad,thomasalrin/etherpad-lite,fread/etherpad-lite,fread/etherpad-lite,simong/etherpad-lite,Gared/etherpad-lite,ktdev/etherpad-lite,pymjer/etherpad-lite,delkyd/etherpad-lite,delkyd/etherpad-lite,coreyaus/etherpad-new,vpistis/etherpad-lite,ether/etherpad-lite,tiblu/etherpad-lite,meticulo3366/etherpad-lite,neynah/etherpad-lite,aptivate/etherpad-lite,bainsy88/etherpad-lite,flavorjones/etherpad-lite-cf,aptivate/etherpad-lite,unnamed89/etherpad,kentonv/etherpad-lite,ktdev/etherpad-lite,meticulo3366/etherpad-lite,syci/etherpad-lite,mozilla/etherpad-lite,DaveAtGit/secpad,Jeff88Ho/etherpad-lite,kenonelah/FactsheetPlugin,Jeff88Ho/etherpad-lite,syci/etherpad-lite,Stackato-Apps/etherpad-lite,ashwini-angular/etherpad-lite,KonradMil/etherpad-lite,simong/etherpad-lite,xavidotron/etherpad-lite,Atober/etherpad-lite,morpheyesh/etherpad-lite,ujdhesa/etherpad-lite,lealife/etherpad-lite,vpistis/etherpad-lite,neynah/etherpad-lite,lealife/etherpad-lite,eXcomm/etherpad-lite,flavorjones/etherpad-lite-cf,syci/etherpad-lite,Stackato-Apps/etherpad-lite,DaveAtGit/secpad,0x46616c6b/etherpad-lite,lpagliari/etherpad-lite,wonderxboy/etherpad-lite,Jeff88Ho/etherpad-lite,webzwo0i/etherpad-lite,skulbrane/etherpad-lite,indykish/etherpad-lite,rschiang/etherpad-lite,thomasrussellmurphy/etherpad-lite,bjonnh/etherpad-lite,cjb/etherpad-lite,mcanthony/etherpad-lite,KonradMil/etherpad-lite,ashwini-angular/etherpad-lite,lealife/etherpad-lite,rschiang/etherpad-lite,unnamed89/etherpad,KonradMil/etherpad-lite,University-of-Potsdam-MM/etherpad-lite,marcstreeter/etherpad-lite,calangohc/etherpad-lite,cloudfoundry-community/etherpad-lite-cf,tudorilisoi/etherpad-lite,leangjia/etherpad-lite,University-of-Potsdam-MM/etherpad-lite,vpistis/etherpad-lite,fourplusone/etherpad-lite,storytouch/etherpad-lite,xavidotron/etherpad-lite,lpagliari/etherpad-lite,viaregio/etherpad-lite,goldquest/etherpad-lite,thomasalrin/etherpad-lite,tudorilisoi/etherpad-lite,mozilla/etherpad-lite,pymjer/etherpad-lite,ocdtrekkie/etherpad-lite,webzwo0i/etherpad-lite,thomasalrin/etherpad-lite,mcanthony/etherpad-lite,tiandishi/wk,ether/etherpad-lite,ErisDS/etherpad-lite,webzwo0i/etherpad-lite,thebestwaytodesignbuildandshipsoftware/etherpad-lite,Jeff88Ho/etherpad-lite,Savi1234/savisql,asac/etherpad-lite,calangohc/etherpad-lite,Atober/etherpad-lite,morpheyesh/etherpad-lite,cdg46/etherpad-lite,joequant/etherpad-lite,indykish/etherpad-lite,flavorjones/etherpad-lite-cf,PuZZleDucK/etherpad-lite,kenonelah/FactsheetPlugin,ErisDS/etherpad-lite,viaregio/etherpad-lite,ErisDS/etherpad-lite,thebestwaytodesignbuildandshipsoftware/etherpad-lite,mani89/etherpad_ncloud,yijunwu/etherpad-lite,girgen/etherpad-lite,emilyxxie/etherpad-lite,bjonnh/etherpad-lite,fourplusone/etherpad-lite,adrienstoffel/ctfpad,ktdev/etherpad-lite,PuZZleDucK/etherpad-lite,nka11/etherpad-lite,kentonv/etherpad-lite,flavorjones/etherpad-lite-cf,bainsy88/etherpad-lite,jumperchen/etherpad-lite,ocdtrekkie/etherpad-lite,aug-riedinger/etherpad,KonradMil/etherpad-lite,Atober/etherpad-lite,PuZZleDucK/etherpad-lite,cmbirk/etherpad-lite,0x46616c6b/etherpad-lite,Gared/etherpad-lite,DaveAtGit/secpad