Fix potential NULL pointer dereference

Fix potential NULL pointer dereference

This patch addresses a potential NULL pointer dereference when we try to
duplicate a string. The input pointer can be NULL and that may lead to
crashes. We simply add a check for that case.

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

	* exec.c (exec_file_locate_attach): Prevent NULL pointer dereference
	when duplicating a string.