x86_64: unbreak test-ptrace
x86_64: unbreak test-ptrace

If dwarf_get returns 0 (indicating the end of call chain), we should
not override the return value to 1. This may result in the caller
continuing to unwind and getting spurious errors.

tony/libunwind,atanasyan/libunwind-android,fdoray/libunwind,dreal-deps/libunwind,libunwind/libunwind,rntz/libunwind,olibc/libunwind,mpercy/libunwind,zeldin/platform_external_libunwind,CyanogenMod/android_external_libunwind,maltek/platform_external_libunwind,vtjnash/libunwind,yuyichao/libunwind,libunwind/libunwind,bo-on-software/libunwind,fdoray/libunwind,unkadoug/libunwind,zliu2014/libunwind-tilegx,android-ia/platform_external_libunwind,dagar/libunwind,CyanogenMod/android_external_libunwind,pathscale/libunwind,CyanogenMod/android_external_libunwind,djwatson/libunwind,evaautomation/libunwind,maltek/platform_external_libunwind,geekboxzone/lollipop_external_libunwind,wdv4758h/libunwind,igprof/libunwind,djwatson/libunwind,Keno/libunwind,tony/libunwind,Keno/libunwind,joyent/libunwind,cloudius-systems/libunwind,cloudius-systems/libunwind,android-ia/platform_external_libunwind,djwatson/libunwind,SyndicateRogue/libunwind,cms-externals/libunwind,vtjnash/libunwind,olibc/libunwind,atanasyan/libunwind,igprof/libunwind,dagar/libunwind,bo-on-software/libunwind,zliu2014/libunwind-tilegx,tkelman/libunwind,frida/libunwind,Chilledheart/libunwind,dreal-deps/libunwind,wdv4758h/libunwind,unkadoug/libunwind,rogwfu/libunwind,cms-externals/libunwind,androidarmv6/android_external_libunwind,zeldin/platform_external_libunwind,rntz/libunwind,martyone/libunwind,android-ia/platform_external_libunwind,frida/libunwind,Chilledheart/libunwind,atanasyan/libunwind,tony/libunwind,atanasyan/libunwind-android,vtjnash/libunwind,tkelman/libunwind,yuyichao/libunwind,Keno/libunwind,rantala/libunwind,krytarowski/libunwind,pathscale/libunwind,vegard/libunwind,Chilledheart/libunwind,martyone/libunwind,tkelman/libunwind,wdv4758h/libunwind,pathscale/libunwind,libunwind/libunwind,martyone/libunwind,vegard/libunwind,DroidSim/platform_external_libunwind,project-zerus/libunwind,joyent/libunwind,dropbox/libunwind,evaautomation/libunwind,fdoray/libunwind,mpercy/libunwind,cloudius-systems/libunwind,atanasyan/libunwind,rogwfu/libunwind,tronical/libunwind,fillexen/libunwind,dagar/libunwind,DroidSim/platform_external_libunwind,zeldin/platform_external_libunwind,SyndicateRogue/libunwind,bo-on-software/libunwind,SyndicateRogue/libunwind,geekboxzone/mmallow_external_libunwind,rntz/libunwind,DroidSim/platform_external_libunwind,igprof/libunwind,androidarmv6/android_external_libunwind,tronical/libunwind,geekboxzone/lollipop_external_libunwind,dropbox/libunwind,mpercy/libunwind,fillexen/libunwind,frida/libunwind,rantala/libunwind,rantala/libunwind,krytarowski/libunwind,cms-externals/libunwind,maltek/platform_external_libunwind,geekboxzone/lollipop_external_libunwind,joyent/libunwind,fillexen/libunwind,unkadoug/libunwind,androidarmv6/android_external_libunwind,geekboxzone/mmallow_external_libunwind,dreal-deps/libunwind,olibc/libunwind,project-zerus/libunwind,dropbox/libunwind,rogwfu/libunwind,evaautomation/libunwind,zliu2014/libunwind-tilegx,krytarowski/libunwind,project-zerus/libunwind,geekboxzone/mmallow_external_libunwind,tronical/libunwind,atanasyan/libunwind-android,vegard/libunwind,yuyichao/libunwind