Fix a very rare crash in GHCi When a BCO with a zero-length bitmap was right at the edge of allocated memory, we were reading a word of non-existent memory.
Fix a very rare crash in GHCi
When a BCO with a zero-length bitmap was right at the edge of
allocated memory, we were reading a word of non-existent memory.

This showed up as a segfault in T789(ghci) for me, but the crash was
extremely sensitive and went away with most changes.

Also, optimised scavenge_large_bitmap a bit while I was in there.

GaloisInc/halvm-ghc,nathyong/microghc-ghc,wxwxwwxxx/ghc,fmthoma/ghc,gridaphobe/ghc,sgillespie/ghc,GaloisInc/halvm-ghc,jstolarek/ghc,ezyang/ghc,nushio3/ghc,AlexanderPankiv/ghc,nushio3/ghc,christiaanb/ghc,shlevy/ghc,oldmanmike/ghc,gcampax/ghc,lukexi/ghc,gridaphobe/ghc,vikraman/ghc,spacekitteh/smcghc,urbanslug/ghc,christiaanb/ghc,AlexanderPankiv/ghc,elieux/ghc,shlevy/ghc,forked-upstream-packages-for-ghcjs/ghc,mcschroeder/ghc,nkaretnikov/ghc,TomMD/ghc,nomeata/ghc,tjakway/ghcjvm,sdiehl/ghc,oldmanmike/ghc,nomeata/ghc,ml9951/ghc,ghc-android/ghc,shlevy/ghc,tibbe/ghc,mettekou/ghc,ghc-android/ghc,nkaretnikov/ghc,ryantm/ghc,lukexi/ghc,wxwxwwxxx/ghc,olsner/ghc,tibbe/ghc,anton-dessiatov/ghc,GaloisInc/halvm-ghc,vikraman/ghc,ghc-android/ghc,forked-upstream-packages-for-ghcjs/ghc,mcschroeder/ghc,AlexanderPankiv/ghc,jstolarek/ghc,gcampax/ghc,oldmanmike/ghc,sgillespie/ghc,spacekitteh/smcghc,da-x/ghc,hferreiro/replay,gridaphobe/ghc,TomMD/ghc,olsner/ghc,sdiehl/ghc,green-haskell/ghc,vikraman/ghc,bitemyapp/ghc,mfine/ghc,jstolarek/ghc,holzensp/ghc,hferreiro/replay,sgillespie/ghc,ghc-android/ghc,wxwxwwxxx/ghc,urbanslug/ghc,siddhanathan/ghc,bitemyapp/ghc,christiaanb/ghc,wxwxwwxxx/ghc,ilyasergey/GHC-XAppFix,mfine/ghc,GaloisInc/halvm-ghc,gcampax/ghc,mfine/ghc,ml9951/ghc,ezyang/ghc,olsner/ghc,ilyasergey/GHC-XAppFix,vTurbine/ghc,mcschroeder/ghc,da-x/ghc,bitemyapp/ghc,tjakway/ghcjvm,anton-dessiatov/ghc,tjakway/ghcjvm,ryantm/ghc,fmthoma/ghc,lukexi/ghc-7.8-arm64,ghc-android/ghc,bitemyapp/ghc,ml9951/ghc,vikraman/ghc,snoyberg/ghc,anton-dessiatov/ghc,mettekou/ghc,forked-upstream-packages-for-ghcjs/ghc,mettekou/ghc,ghc-android/ghc,acowley/ghc,olsner/ghc,siddhanathan/ghc,mfine/ghc,siddhanathan/ghc,forked-upstream-packages-for-ghcjs/ghc,fmthoma/ghc,christiaanb/ghc,oldmanmike/ghc,vTurbine/ghc,spacekitteh/smcghc,siddhanathan/ghc,urbanslug/ghc,vTurbine/ghc,gridaphobe/ghc,nomeata/ghc,elieux/ghc,snoyberg/ghc,gridaphobe/ghc,frantisekfarka/ghc-dsi,nushio3/ghc,vTurbine/ghc,vikraman/ghc,TomMD/ghc,mettekou/ghc,ekmett/ghc,ml9951/ghc,ml9951/ghc,snoyberg/ghc,hferreiro/replay,anton-dessiatov/ghc,AlexanderPankiv/ghc,nathyong/microghc-ghc,forked-upstream-packages-for-ghcjs/ghc,oldmanmike/ghc,da-x/ghc,sdiehl/ghc,olsner/ghc,ekmett/ghc,anton-dessiatov/ghc,anton-dessiatov/ghc,mcmaniac/ghc,vikraman/ghc,wxwxwwxxx/ghc,vTurbine/ghc,siddhanathan/ghc,AlexanderPankiv/ghc,mcmaniac/ghc,sgillespie/ghc,nkaretnikov/ghc,tjakway/ghcjvm,sdiehl/ghc,lukexi/ghc,vikraman/ghc,hferreiro/replay,ml9951/ghc,ezyang/ghc,nathyong/microghc-ghc,sgillespie/ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,urbanslug/ghc,gcampax/ghc,tibbe/ghc,holzensp/ghc,mcschroeder/ghc,da-x/ghc,mettekou/ghc,mettekou/ghc,tjakway/ghcjvm,nkaretnikov/ghc,GaloisInc/halvm-ghc,ekmett/ghc,fmthoma/ghc,AlexanderPankiv/ghc,urbanslug/ghc,gridaphobe/ghc,da-x/ghc,ezyang/ghc,tjakway/ghcjvm,ml9951/ghc,sdiehl/ghc,ilyasergey/GHC-XAppFix,hferreiro/replay,nathyong/microghc-ghc,mcmaniac/ghc,frantisekfarka/ghc-dsi,elieux/ghc,ryantm/ghc,TomMD/ghc,green-haskell/ghc,lukexi/ghc-7.8-arm64,elieux/ghc,mettekou/ghc,acowley/ghc,fmthoma/ghc,acowley/ghc,frantisekfarka/ghc-dsi,snoyberg/ghc,green-haskell/ghc,ryantm/ghc,snoyberg/ghc,sgillespie/ghc,nushio3/ghc,nushio3/ghc,tjakway/ghcjvm,vTurbine/ghc,forked-upstream-packages-for-ghcjs/ghc,nomeata/ghc,lukexi/ghc,sdiehl/ghc,nkaretnikov/ghc,da-x/ghc,acowley/ghc,nomeata/ghc,urbanslug/ghc,lukexi/ghc-7.8-arm64,ekmett/ghc,nkaretnikov/ghc,gcampax/ghc,bitemyapp/ghc,mcschroeder/ghc,holzensp/ghc,mcmaniac/ghc,wxwxwwxxx/ghc,nathyong/microghc-ghc,sgillespie/ghc,vTurbine/ghc,tibbe/ghc,TomMD/ghc,nushio3/ghc,shlevy/ghc,snoyberg/ghc,lukexi/ghc-7.8-arm64,AlexanderPankiv/ghc,christiaanb/ghc,sdiehl/ghc,mfine/ghc,ml9951/ghc,ghc-android/ghc,frantisekfarka/ghc-dsi,olsner/ghc,acowley/ghc,fmthoma/ghc,da-x/ghc,elieux/ghc,mcschroeder/ghc,elieux/ghc,mcschroeder/ghc,hferreiro/replay,nushio3/ghc,ilyasergey/GHC-XAppFix,nathyong/microghc-ghc,jstolarek/ghc,mfine/ghc,TomMD/ghc,green-haskell/ghc,christiaanb/ghc,holzensp/ghc,jstolarek/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,frantisekfarka/ghc-dsi,oldmanmike/ghc,shlevy/ghc,anton-dessiatov/ghc,olsner/ghc,gridaphobe/ghc,ekmett/ghc,ryantm/ghc,wxwxwwxxx/ghc,nkaretnikov/ghc,gcampax/ghc,mcmaniac/ghc,lukexi/ghc-7.8-arm64,ezyang/ghc,mfine/ghc,spacekitteh/smcghc,ezyang/ghc,tibbe/ghc,green-haskell/ghc,forked-upstream-packages-for-ghcjs/ghc,TomMD/ghc,fmthoma/ghc,snoyberg/ghc,christiaanb/ghc,siddhanathan/ghc,spacekitteh/smcghc,hferreiro/replay,elieux/ghc,siddhanathan/ghc,oldmanmike/ghc,gcampax/ghc,urbanslug/ghc,acowley/ghc,acowley/ghc,holzensp/ghc,nathyong/microghc-ghc,lukexi/ghc