Avoid "may be used uninitialized" warning

Hi,
I see such warning below on one compiler I am using.

cc1: warnings being treated as errors
../../workspace/gdb/stack.c: In function 'frame_info':
../../workspace/gdb/stack.c:1519:20: error: 'caller_pc' may be used uninitialized in this function

Go through the gdb-patches archives and find the "canonical" way to
fix this warning is to initialize the variable.

gdb:

2013-12-08  Yao Qi  <yao@codesourcery.com>

	* stack.c (frame_info): Initialize variable caller_pc.