Change reopen_exec_file to check result of stat

This seems to be a real bug found by -Wunused-but-set-variable.  If
"stat" fails for some reason, gdb would use the uninitialized "st".

2016-07-14  Tom Tromey  <tom@tromey.com>

	* corefile.c (reopen_exec_file): Only examine st.st_mtime if stat
	succeeded.