[sanitizer] Stop unwinding the stack when a close-to-zero PC is found

On OS X, we often get stack trace in a report that ends with a 0x0 frame.  To get rid of it, let's trim the stack trace when we find a close-to-zero value, which is obviously not a valid PC.

Differential Revision: http://reviews.llvm.org/D14656

llvm-svn: 273886