submodule deinit: handle non existing pathspecs gracefully

This fixes a regression introduced in 2e612731 (submodule: port
submodule subcommand 'deinit' from shell to C, 2018-01-15), when
handling pathspecs that do not exist gracefully. This restores the
historic behavior of reporting the pathspec as unknown and returning
instead of reporting a bug.

Reported-by: Peter Oberndorfer <kumbayo84@arcor.de>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/submodule--helper.c

@@ -1042,7 +1042,7 @@ static int module_deinit(int argc, const char **argv, const char *prefix)
 		die(_("Use '--all' if you really want to deinitialize all submodules"));
 	if (module_list_compute(argc, argv, prefix, &pathspec, &list) < 0)
-		BUG("module_list_compute should not choke on empty pathspec");
+		return 1;
 	info.prefix = prefix;
 	if (quiet)