push: defensive programming

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

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

builtin/push.c

@@ -90,7 +90,7 @@ static void refspec_append_mapped(struct refspec *refspec, const char *ref,
 	if (push_default == PUSH_DEFAULT_UPSTREAM &&
 	    skip_prefix(matched->name, "refs/heads/", &branch_name)) {
 		struct branch *branch = branch_get(branch_name);
-		if (branch->merge_nr == 1 && branch->merge[0]->src) {
+		if (branch && branch->merge_nr == 1 && branch->merge[0]->src) {
 			refspec_appendf(refspec, "%s:%s",
 					ref, branch->merge[0]->src);
 			return;