halloc: Cast the offsetof macro to ptrdiff_t before negating
halloc: Cast the offsetof macro to ptrdiff_t before negating

This gets rid of the warning
"C4146: unary minus applied to unsigned type, result still unsigned"
with visual c++.

Change-Id: I6eb24da983136d798221db4d3a5f50dc2857a03b

turbulenz/libvpx,Acidburn0zzz/webm.libvpx,webmproject/libvpx,jacklicn/webm.libvpx,VTCSecureLLC/libvpx,Laknot/libvpx,matanbs/webm.libvpx,ittiamvpx/libvpx-1,cinema6/libvpx,ShiftMediaProject/libvpx,matanbs/vp982,Distrotech/libvpx,reimaginemedia/webm.libvpx,Laknot/libvpx,mbebenita/aom,pcwalton/libvpx,luctrudeau/aom,Maria1099/webm.libvpx,Suvarna1488/webm.libvpx,reimaginemedia/webm.libvpx,lyx2014/libvpx_c,ShiftMediaProject/libvpx,GrokImageCompression/aom,GrokImageCompression/aom,ittiamvpx/libvpx,charup/https---github.com-webmproject-libvpx-,kalli123/webm.libvpx,shyamalschandra/libvpx,charup/https---github.com-webmproject-libvpx-,VTCSecureLLC/libvpx,Suvarna1488/webm.libvpx,thdav/aom,Acidburn0zzz/webm.libvpx,GrokImageCompression/aom,charup/https---github.com-webmproject-libvpx-,shareefalis/libvpx,running770/libvpx,mbebenita/aom,webmproject/libvpx,thdav/aom,n4t/libvpx,shareefalis/libvpx,openpeer/libvpx_new,jdm/libvpx,shyamalschandra/libvpx,kalli123/webm.libvpx,WebRTC-Labs/libvpx,mwgoldsmith/libvpx,stewnorriss/libvpx,gshORTON/webm.libvpx,mbebenita/aom,charup/https---github.com-webmproject-libvpx-,goodleixiao/vpx,gshORTON/webm.libvpx,smarter/aom,turbulenz/libvpx,altogother/webm.libvpx,jdm/libvpx,Maria1099/webm.libvpx,lyx2014/libvpx_c,Topopiccione/libvpx,jmvalin/aom,shacklettbp/aom,kalli123/webm.libvpx,zofuthan/libvpx,Suvarna1488/webm.libvpx,WebRTC-Labs/libvpx,stewnorriss/libvpx,Maria1099/webm.libvpx,shacklettbp/aom,cinema6/libvpx,gshORTON/webm.libvpx,WebRTC-Labs/libvpx,iniwf/webm.libvpx,jdm/libvpx,jdm/libvpx,reimaginemedia/webm.libvpx,reimaginemedia/webm.libvpx,zofuthan/libvpx,mwgoldsmith/libvpx,GrokImageCompression/aom,lyx2014/libvpx_c,jacklicn/webm.libvpx,hsueceumd/test_hui,altogother/webm.libvpx,turbulenz/libvpx,liqianggao/libvpx,liqianggao/libvpx,matanbs/vp982,Acidburn0zzz/webm.libvpx,Topopiccione/libvpx,ittiamvpx/libvpx,shareefalis/libvpx,shareefalis/libvpx,Topopiccione/libvpx,mwgoldsmith/vpx,abwiz0086/webm.libvpx,cinema6/libvpx,kleopatra999/webm.libvpx,cinema6/libvpx,running770/libvpx,altogother/webm.libvpx,Distrotech/libvpx,VTCSecureLLC/libvpx,Suvarna1488/webm.libvpx,running770/libvpx,luctrudeau/aom,shareefalis/libvpx,goodleixiao/vpx,turbulenz/libvpx,abwiz0086/webm.libvpx,matanbs/webm.libvpx,matanbs/webm.libvpx,goodleixiao/vpx,ittiamvpx/libvpx-1,stewnorriss/libvpx,Distrotech/libvpx,abwiz0086/webm.libvpx,webmproject/libvpx,matanbs/vp982,abwiz0086/webm.libvpx,Distrotech/libvpx,shyamalschandra/libvpx,stewnorriss/libvpx,ShiftMediaProject/libvpx,mbebenita/aom,pcwalton/libvpx,sanyaade-teachings/libvpx,Suvarna1488/webm.libvpx,pcwalton/libvpx,reimaginemedia/webm.libvpx,ShiftMediaProject/libvpx,reimaginemedia/webm.libvpx,Laknot/libvpx,shacklettbp/aom,mbebenita/aom,shacklettbp/aom,zofuthan/libvpx,Distrotech/libvpx,jmvalin/aom,charup/https---github.com-webmproject-libvpx-,abwiz0086/webm.libvpx,turbulenz/libvpx,mwgoldsmith/vpx,mbebenita/aom,mwgoldsmith/libvpx,mbebenita/aom,felipebetancur/libvpx,cinema6/libvpx,kim42083/webm.libvpx,goodleixiao/vpx,mwgoldsmith/vpx,sanyaade-teachings/libvpx,stewnorriss/libvpx,ittiamvpx/libvpx,smarter/aom,hsueceumd/test_hui,luctrudeau/aom,Laknot/libvpx,shareefalis/libvpx,smarter/aom,liqianggao/libvpx,Acidburn0zzz/webm.libvpx,Topopiccione/libvpx,ittiamvpx/libvpx,kleopatra999/webm.libvpx,iniwf/webm.libvpx,VTCSecureLLC/libvpx,shyamalschandra/libvpx,mwgoldsmith/vpx,Acidburn0zzz/webm.libvpx,running770/libvpx,thdav/aom,ShiftMediaProject/libvpx,shyamalschandra/libvpx,openpeer/libvpx_new,matanbs/vp982,jdm/libvpx,jacklicn/webm.libvpx,zofuthan/libvpx,felipebetancur/libvpx,jmvalin/aom,GrokImageCompression/aom,Suvarna1488/webm.libvpx,smarter/aom,kalli123/webm.libvpx,lyx2014/libvpx_c,kleopatra999/webm.libvpx,hsueceumd/test_hui,gshORTON/webm.libvpx,mbebenita/aom,Topopiccione/libvpx,kim42083/webm.libvpx,turbulenz/libvpx,running770/libvpx,Distrotech/libvpx,iniwf/webm.libvpx,matanbs/vp982,thdav/aom,shyamalschandra/libvpx,zofuthan/libvpx,altogother/webm.libvpx,hsueceumd/test_hui,n4t/libvpx,felipebetancur/libvpx,kleopatra999/webm.libvpx,WebRTC-Labs/libvpx,gshORTON/webm.libvpx,ittiamvpx/libvpx-1,jacklicn/webm.libvpx,VTCSecureLLC/libvpx,altogother/webm.libvpx,iniwf/webm.libvpx,n4t/libvpx,gshORTON/webm.libvpx,thdav/aom,Laknot/libvpx,jacklicn/webm.libvpx,ittiamvpx/libvpx-1,kalli123/webm.libvpx,liqianggao/libvpx,iniwf/webm.libvpx,mwgoldsmith/libvpx,kleopatra999/webm.libvpx,jacklicn/webm.libvpx,shacklettbp/aom,ittiamvpx/libvpx-1,Maria1099/webm.libvpx,webmproject/libvpx,ittiamvpx/libvpx-1,WebRTC-Labs/libvpx,matanbs/vp982,liqianggao/libvpx,felipebetancur/libvpx,cinema6/libvpx,pcwalton/libvpx,goodleixiao/vpx,Topopiccione/libvpx,mwgoldsmith/vpx,pcwalton/libvpx,webmproject/libvpx,jmvalin/aom,openpeer/libvpx_new,hsueceumd/test_hui,Acidburn0zzz/webm.libvpx,Maria1099/webm.libvpx,turbulenz/libvpx,ittiamvpx/libvpx,smarter/aom,zofuthan/libvpx,Laknot/libvpx,running770/libvpx,matanbs/webm.libvpx,altogother/webm.libvpx,openpeer/libvpx_new,stewnorriss/libvpx,matanbs/vp982,turbulenz/libvpx,liqianggao/libvpx,jmvalin/aom,hsueceumd/test_hui,abwiz0086/webm.libvpx,n4t/libvpx,ittiamvpx/libvpx,smarter/aom,kim42083/webm.libvpx,GrokImageCompression/aom,thdav/aom,mwgoldsmith/libvpx,luctrudeau/aom,Maria1099/webm.libvpx,felipebetancur/libvpx,luctrudeau/aom,openpeer/libvpx_new,luctrudeau/aom,jmvalin/aom,kim42083/webm.libvpx,mwgoldsmith/libvpx,felipebetancur/libvpx,kalli123/webm.libvpx,webmproject/libvpx,lyx2014/libvpx_c,sanyaade-teachings/libvpx,charup/https---github.com-webmproject-libvpx-,cinema6/libvpx,kim42083/webm.libvpx,mwgoldsmith/vpx,n4t/libvpx,pcwalton/libvpx,kim42083/webm.libvpx,lyx2014/libvpx_c,jdm/libvpx,iniwf/webm.libvpx,sanyaade-teachings/libvpx,turbulenz/libvpx,goodleixiao/vpx,openpeer/libvpx_new,matanbs/webm.libvpx,sanyaade-teachings/libvpx,mbebenita/aom,kleopatra999/webm.libvpx,matanbs/webm.libvpx,VTCSecureLLC/libvpx,shacklettbp/aom