Fix when walking stack when no module list is present and the return address has to be scanned.
Fix when walking stack when no module list is present and the return address has to be
scanned.

http://breakpad.appspot.com/36002

R=doshimun
A=nealsid



git-svn-id: a14e546401035be51a469fbea25ecce70a1a22f2@420 4c0a9323-5329-0410-9bdc-e9ce6186880e

tomgr/google-breakpad,tomgr/google-breakpad,tomgr/google-breakpad,tomgr/google-breakpad,tomgr/google-breakpad