fix #15973
fix #15973

Rework of stream_seek handling; there where basically two bugs: 1. seeking to the end of the current file would fail (with SEEK_SET); and 2. if seeking to an undefined position (outside 0,unencryptedSize) then newPosition was not defined. I used the opportunity to simplify the code.
cernbox/core,pollopolea/core,nextcloud/server,endsguy/server,owncloud/core,jbicha/server,xx621998xx/server,michaelletzgus/nextcloud-server,pixelipo/server,lrytz/core,whitekiba/server,lrytz/core,whitekiba/server,owncloud/core,sharidas/core,endsguy/server,andreas-p/nextcloud-server,andreas-p/nextcloud-server,IljaN/core,cernbox/core,jbicha/server,lrytz/core,xx621998xx/server,michaelletzgus/nextcloud-server,sharidas/core,bluelml/core,owncloud/core,Ardinis/server,pmattern/server,IljaN/core,jbicha/server,cernbox/core,xx621998xx/server,nextcloud/server,IljaN/core,Ardinis/server,pollopolea/core,bluelml/core,pollopolea/core,pollopolea/core,whitekiba/server,pixelipo/server,Ardinis/server,jbicha/server,sharidas/core,whitekiba/server,endsguy/server,pmattern/server,andreas-p/nextcloud-server,bluelml/core,IljaN/core,xx621998xx/server,Ardinis/server,whitekiba/server,owncloud/core,andreas-p/nextcloud-server,cernbox/core,bluelml/core,pixelipo/server,sharidas/core,sharidas/core,pmattern/server,michaelletzgus/nextcloud-server,pollopolea/core,pmattern/server,lrytz/core,pixelipo/server,bluelml/core,lrytz/core,Ardinis/server,jbicha/server,andreas-p/nextcloud-server,phil-davis/core,cernbox/core,pmattern/server,xx621998xx/server,owncloud/core,nextcloud/server,IljaN/core,endsguy/server,endsguy/server,pixelipo/server,michaelletzgus/nextcloud-server,nextcloud/server