Remove cleanup from add_path

Remove cleanup from add_path

This removes a cleanup from add_path, replacing it with a use of
gdb::unique_xmalloc_ptr.  Note that this declaration had to be hoisted
somewhat, to avoid inteference from the "goto"s in this function.

gdb/ChangeLog
2018-09-13  Tom Tromey  <tom@tromey.com>

	* source.c (add_path): Use gdb::unique_xmalloc_ptr.