Stop unwinding on undefined return address
Stop unwinding on undefined return address

Milian Wolff <mail@milianw.de> reported that this greatly
improved unwinding perf on some his apps.

vegard/libunwind,libunwind/libunwind,djwatson/libunwind,Chilledheart/libunwind,krytarowski/libunwind,pathscale/libunwind,tkelman/libunwind,igprof/libunwind,yuyichao/libunwind,Chilledheart/libunwind,dagar/libunwind,martyone/libunwind,djwatson/libunwind,evaautomation/libunwind,fdoray/libunwind,wdv4758h/libunwind,unkadoug/libunwind,yuyichao/libunwind,bo-on-software/libunwind,rntz/libunwind,joyent/libunwind,pathscale/libunwind,pathscale/libunwind,project-zerus/libunwind,vegard/libunwind,fdoray/libunwind,vtjnash/libunwind,krytarowski/libunwind,Chilledheart/libunwind,libunwind/libunwind,evaautomation/libunwind,wdv4758h/libunwind,tony/libunwind,rntz/libunwind,frida/libunwind,frida/libunwind,dropbox/libunwind,tkelman/libunwind,unkadoug/libunwind,atanasyan/libunwind,igprof/libunwind,wdv4758h/libunwind,tony/libunwind,tony/libunwind,yuyichao/libunwind,rantala/libunwind,joyent/libunwind,project-zerus/libunwind,djwatson/libunwind,vegard/libunwind,atanasyan/libunwind,dropbox/libunwind,rantala/libunwind,unkadoug/libunwind,evaautomation/libunwind,martyone/libunwind,project-zerus/libunwind,bo-on-software/libunwind,rantala/libunwind,vtjnash/libunwind,bo-on-software/libunwind,vtjnash/libunwind,fdoray/libunwind,joyent/libunwind,dagar/libunwind,dropbox/libunwind,krytarowski/libunwind,dagar/libunwind,martyone/libunwind,tkelman/libunwind,igprof/libunwind,atanasyan/libunwind,rntz/libunwind,libunwind/libunwind,frida/libunwind