mgcp_test: fix possible double free

Change-Id: I2d044382c0bb66e190400e3397449c3f2387359a
diff --git a/tests/mgcp/mgcp_test.c b/tests/mgcp/mgcp_test.c
index ee49eef..330d24d 100644
--- a/tests/mgcp/mgcp_test.c
+++ b/tests/mgcp/mgcp_test.c
@@ -685,7 +685,8 @@
 				printf("(response does not contain a connection id)\n");
 		}
 
-		msgb_free(msg);
+		if (msg)
+			msgb_free(msg);
 
 		if (dummy_packets)
 			printf("Dummy packets: %d\n", dummy_packets);