Fix BFD leak in solib-darwin.c

Fix BFD leak in solib-darwin.c

commit 192b62ce0b4bb5c61188f570e127a26d2c32f716 ("Use class to manage
BFD reference counts") changed darwin_get_dyld_bfd to use:

+	dyld_bfd.release ();

rather than

-      do_cleanups (cleanup);

However, using release here leaks the BFD.  Instead, simply assigning
"sub" to "dyld_bfd" achieves what was meant.

gdb/ChangeLog
2019-02-25  Tom Tromey  <tromey@adacore.com>

	* solib-darwin.c (darwin_get_dyld_bfd): Don't release dyld_bfd.