Unwind: prevent unw_get_proc_info from returning stale data

If unwind info is not available at the current IP, unw_get_proc_info should
return a zero-filled structure rather than the info of the previous IP.

This change also makes unw_get_proc_info return UNW_ENOINFO instead of
UNW_ESUCCESS.

Patch by Amanieu d'Antras!

llvm-svn: 372407