submodule: fix submodule names depending on config-owned memory

When populating the list of submodule names, we use the submodule
configuration entry's name as the key in the map of submodule names.
This creates a hidden dependency on the liveliness of the configuration
that was used to parse the submodule, which is fragile and unexpected.

Fix the issue by duplicating the string before writing it into the
submodule name map.

src/submodule.c

@@ -199,13 +199,15 @@ out:
  */
 static void free_submodule_names(git_strmap *names)
 {
-	git_buf *name;
+	const char *key;
+	char *value;
 
 	if (names == NULL)
 		return;
 
-	git_strmap_foreach_value(names, name, {
-		git__free(name);
+	git_strmap_foreach(names, key, value, {
+		git__free((char *) key);
+		git__free(value);
 	});
 	git_strmap_free(names);
 
@@ -257,7 +259,7 @@ static int load_submodule_names(git_strmap **out, git_repository *repo, git_conf
 		if (!isvalid)
 			continue;
 
-		git_strmap_insert(names, entry->value, git_buf_detach(&buf), &rval);
+		git_strmap_insert(names, git__strdup(entry->value), git_buf_detach(&buf), &rval);
 		if (rval < 0) {
 			giterr_set(GITERR_NOMEMORY, "error inserting submodule into hash table");
 			error = -1;