submodule: check for NULL return of get_submodule_ref_store()

If we can't find a ref store for a submodule then assume the latter
is not initialized (or was removed).  Print a status line accordingly
instead of causing a segmentation fault by passing NULL as the first
parameter of refs_head_ref().

Reported-by: Jeremy Feusi <jeremy@feusi.co>
Reviewed-by: Stefan Beller <sbeller@google.com>
Initial-Test-By: Stefan Beller <sbeller@google.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/submodule--helper.c

@@ -620,9 +620,13 @@ static void status_submodule(const char *path, const struct object_id *ce_oid,
 			     displaypath);
 	} else if (!(flags & OPT_CACHED)) {
 		struct object_id oid;
+		struct ref_store *refs = get_submodule_ref_store(path);
 
-		if (refs_head_ref(get_submodule_ref_store(path),
-				  handle_submodule_head_ref, &oid))
+		if (!refs) {
+			print_status(flags, '-', path, ce_oid, displaypath);
+			goto cleanup;
+		}
+		if (refs_head_ref(refs, handle_submodule_head_ref, &oid))
 			die(_("could not resolve HEAD ref inside the "
 			      "submodule '%s'"), path);
 

t/t7400-submodule-basic.sh

@@ -821,6 +821,21 @@ test_expect_success 'moving the superproject does not break submodules' '
 	)
 '
 
+test_expect_success 'moving the submodule does not break the superproject' '
+	(
+		cd addtest2 &&
+		git submodule status
+	) >actual &&
+	sed -e "s/^ \([^ ]* repo\) .*/-\1/" <actual >expect &&
+	mv addtest2/repo addtest2/repo.bak &&
+	test_when_finished "mv addtest2/repo.bak addtest2/repo" &&
+	(
+		cd addtest2 &&
+		git submodule status
+	) >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'submodule add --name allows to replace a submodule with another at the same path' '
 	(
 		cd addtest2 &&