Exit unwinding early when at the top of the stack and additional info won't be found.

Summary:
This patch follows libgcc's lead: When the return-address register is
zero, there won't be additional stack frames to examine, or gather
information about. Exit before spending time looking for something
known not to be found.

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77099