MFC r242943: Do not unwind past a zero PC frame.

MFC r242943: Do not unwind past a zero PC frame.

  This improves GDB usability when debugging code compiled with
  optimization.

  Upstream GDB revision f0031b6d3ae9b164b3747986ab898190bd4dcf8c (prior
  to GDB's switch to GPLv3), with frame_debug_got_null_frame expanded
  inline.

  | 2004-12-12  Andrew Cagney  <cagney@gnu.org>
  |
  | * frame.c (get_prev_frame): When unwinding normal frames, check
  | that the PC isn't zero.

Sponsored by:	ADARA Networks

git-svn-id: svn://svn.freebsd.org/base/stable/8@244119 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f