Avoid potential memory leak in find_frame_funname

Avoid potential memory leak in find_frame_funname

The PR 18565 thread pointed out that, if cp_remove_params can throw
(we aren't quite sure), then find_frame_funname could leak some
memory.  This patch avoids any potential issue by rearranging some
code in find_frame_funname.

Built and regtested on x86-64 Fedora 24.

2016-08-03  Tom Tromey  <tom@tromey.com>

	* stack.c (find_frame_funname): Avoid any possible leak in case
	cp_remove_params can throw.