fetch: defensive programming

CodeQL points out that `branch_get()` can return NULL values.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

builtin/fetch.c

@@ -552,7 +552,7 @@ static struct ref *get_ref_map(struct remote *remote,
 		if (remote &&
 		    (remote->fetch.nr ||
 		     /* Note: has_merge implies non-NULL branch->remote_name */
-		     (has_merge && !strcmp(branch->remote_name, remote->name)))) {
+		     (has_merge && branch && !strcmp(branch->remote_name, remote->name)))) {
 			for (i = 0; i < remote->fetch.nr; i++) {
 				get_fetch_map(remote_refs, &remote->fetch.items[i], &tail, 0);
 				if (remote->fetch.items[i].dst &&
@@ -570,6 +570,7 @@ static struct ref *get_ref_map(struct remote *remote,
 			 * Note: has_merge implies non-NULL branch->remote_name
 			 */
 			if (has_merge &&
+			    branch &&
 			    !strcmp(branch->remote_name, remote->name))
 				add_merge_config(&ref_map, remote_refs, branch, &tail);
 		} else if (!prefetch) {