folly/Format-inl.h: use the correct type for fwrite return value (easy)
folly/Format-inl.h: use the correct type for fwrite return value (easy)

Summary:
* folly/Format-inl.h (folly): fwrite returns size_t, not ssize_t.
Fixing this avoids the following diagnostic from gcc-4.9:
folly/Format-inl.h:299:11: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]

Test Plan:
Run this and note there are fewer errors than before:
fbconfig --platform-all=gcc-4.9-glibc-2.20 -r folly && fbmake dbgo

Reviewed By: philipp@fb.com

Subscribers: trunkagent, net-systems@, folly-diffs@

FB internal diff: D1770155

Tasks: 5941250

Signature: t1:1770155:1420668733:62593231d35f5874521dba575f23b68891b2a4f9

xzmagic/folly,brunomorishita/folly,constantine001/folly,reddit/folly,gavioto/folly,bobegir/folly,rklabs/folly,loverszhaokai/folly,fw1121/folly,facebook/folly,leolujuyi/folly,renyinew/folly,raphaelamorim/folly,gaoyingie/folly,doctaweeks/folly,loversInJapan/folly,mqeizi/folly,SeanRBurton/folly,kernelim/folly,chjp2046/folly,fw1121/folly,project-zerus/folly,Orvid/folly,arg0/folly,project-zerus/folly,renyinew/folly,clearlylin/folly,guker/folly,tomhughes/folly,mqeizi/folly,upsoft/folly,theiver9827/folly,sonnyhu/folly,CJstar/folly,leolujuyi/folly,charsyam/folly,hongliangzhao/folly,kernelim/folly,gaoyingie/folly,tomhughes/folly,ykarlsbrun/folly,sakishum/folly,zhiweicai/folly,ykarlsbrun/folly,bowlofstew/folly,romange/folly,upsoft/folly,chjp2046/folly,leolujuyi/folly,tempbottle/folly,SammyK/folly,nickhen/folly,SammyK/folly,stonegithubs/folly,mqeizi/folly,chjp2046/folly,charsyam/folly,lifei/folly,guker/folly,doctaweeks/folly,PoisonBOx/folly,facebook/folly,loversInJapan/folly,sonnyhu/folly,mqeizi/folly,juniway/folly,renyinew/folly,SammyK/folly,gaoyingie/folly,zhiweicai/folly,doctaweeks/folly,PoisonBOx/folly,bsampath/folly,kernelim/folly,PoisonBOx/folly,arg0/folly,guker/folly,tempbottle/folly,sonnyhu/folly,doctaweeks/folly,reddit/folly,theiver9827/folly,shaobz/folly,alexst07/folly,fw1121/folly,tomhughes/folly,facebook/folly,Orvid/folly,leolujuyi/folly,Hincoin/folly,guker/folly,alexst07/folly,gavioto/folly,constantine001/folly,loversInJapan/folly,brunomorishita/folly,raphaelamorim/folly,SammyK/folly,Hincoin/folly,loverszhaokai/folly,SeanRBurton/folly,CJstar/folly,guker/folly,Orvid/folly,zhiweicai/folly,romange/folly,Eagle-X/folly,SammyK/folly,brunomorishita/folly,project-zerus/folly,lifei/folly,rklabs/folly,clearlinux/folly,yangjin-unique/folly,cole14/folly,arg0/folly,zhiweicai/folly,SeanRBurton/folly,bobegir/folly,bowlofstew/folly,hongliangzhao/folly,gavioto/folly,hongliangzhao/folly,KSreeHarsha/folly,juniway/folly,xzmagic/folly,upsoft/folly,yangjin-unique/folly,sonnyhu/folly,bobegir/folly,clearlylin/folly,arg0/folly,cole14/folly,tempbottle/folly,KSreeHarsha/folly,hongliangzhao/folly,sakishum/folly,alexst07/folly,nickhen/folly,bsampath/folly,charsyam/folly,rklabs/folly,sonnyhu/folly,KSreeHarsha/folly,xzmagic/folly,Hincoin/folly,constantine001/folly,gavioto/folly,raphaelamorim/folly,constantine001/folly,juniway/folly,brunomorishita/folly,stonegithubs/folly,juniway/folly,cppfool/folly,shaobz/folly,Eagle-X/folly,reddit/folly,bikong2/folly,SeanRBurton/folly,rklabs/folly,facebook/folly,stonegithubs/folly,fw1121/folly,xzmagic/folly,Orvid/folly,loversInJapan/folly,shaobz/folly,arg0/folly,shaobz/folly,bowlofstew/folly,raphaelamorim/folly,Hincoin/folly,doctaweeks/folly,bobegir/folly,clearlylin/folly,floxard/folly,Orvid/folly,renyinew/folly,colemancda/folly,bowlofstew/folly,PPC64/folly,tomhughes/folly,ykarlsbrun/folly,PPC64/folly,CJstar/folly,project-zerus/folly,cppfool/folly,theiver9827/folly,cppfool/folly,yangjin-unique/folly,chjp2046/folly,cole14/folly,hongliangzhao/folly,PoisonBOx/folly,wildinto/folly,upsoft/folly,colemancda/folly,colemancda/folly,loverszhaokai/folly,PoisonBOx/folly,fw1121/folly,ykarlsbrun/folly,sakishum/folly,theiver9827/folly,floxard/folly,tempbottle/folly,chjp2046/folly,bowlofstew/folly,gaoyingie/folly,loverszhaokai/folly,reddit/folly,theiver9827/folly,renyinew/folly,brunomorishita/folly,KSreeHarsha/folly,lifei/folly,shaobz/folly,xzmagic/folly,zhiweicai/folly,bobegir/folly,colemancda/folly,yangjin-unique/folly,sakishum/folly,romange/folly,gavioto/folly,mqeizi/folly,SeanRBurton/folly,CJstar/folly,colemancda/folly,cppfool/folly,nickhen/folly,charsyam/folly,kernelim/folly,PPC64/folly,wildinto/folly,upsoft/folly,Eagle-X/folly,tomhughes/folly,reddit/folly,stonegithubs/folly,bsampath/folly,stonegithubs/folly,project-zerus/folly,bikong2/folly,PPC64/folly,cppfool/folly,floxard/folly,alexst07/folly,nickhen/folly,wildinto/folly,bsampath/folly,Eagle-X/folly,bikong2/folly,clearlylin/folly,nickhen/folly,lifei/folly,clearlinux/folly,lifei/folly,CJstar/folly,yangjin-unique/folly,clearlinux/folly,KSreeHarsha/folly,gaoyingie/folly,floxard/folly,bsampath/folly,rklabs/folly,romange/folly,sakishum/folly,cole14/folly,clearlinux/folly,bikong2/folly,Eagle-X/folly,cole14/folly,loverszhaokai/folly,constantine001/folly,kernelim/folly,Hincoin/folly,bikong2/folly,facebook/folly,romange/folly,leolujuyi/folly,raphaelamorim/folly,floxard/folly,PPC64/folly,ykarlsbrun/folly,loversInJapan/folly,wildinto/folly,juniway/folly,clearlylin/folly,tempbottle/folly,alexst07/folly,wildinto/folly,clearlinux/folly