[libunwind] Fix unw_get_proc_info sometimes returning stale data

[Amanieu]

Summary

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.

[compnerd]

Would be nice if we could have a test case for this.

[mstorsjo]

Can the runtimes repo maintainers @mclow.lists @ldionne @EricWF give this an ack aswell?

@Amanieu Do you need someone to commit this for you?

[Amanieu]

Added a test.

[Amanieu]

@mstorsjo Yes, I need someone to commit this for me.

[compnerd]

SVN r372407

libunwind/src/UnwindCursor.hpp

 
 template <typename A, typename R>
 void UnwindCursor<A, R>::getInfo(unw_proc_info_t *info) {
-  *info = _info;
+  if (_unwindInfoMissing)
+    memset(info, 0, sizeof(*info));
+  else
+    *info = _info;
 }
 
 template <typename A, typename R>