Fix obvious gotcha in string comparison

Fix obvious gotcha in string comparison

This patch fixes a gotcha when comparing exception's messages in
exception_print_same. It should've used the statically-allocated
string versions msg1 and msg2 instead.

As is, it could lead to crashes.

gdb/ChangeLog:
2016-10-24  Luis Machado  <lgustavo@codesourcery.com>

	* exec.c (exception_print_same): Fix string comparison to use
	statically-allocated ones.