res_pjsip_outbound_publish: Potential crash due to off nominal path

It was possible for the explicit publish destroy function to be called without
the pjsip client ever being initialized. This fix checks to make sure there is
a client to destroy before attempting.

Change-Id: I8eea1bfa3bd472149bfc255310be2a6248688f5c

res/res_pjsip_outbound_publish.c

@@ -687,8 +687,15 @@ static int explicit_publish_destroy(void *data)
 {
 	struct ast_sip_outbound_publish_client *client = data;
 
-	pjsip_publishc_destroy(client->client);
-	ao2_ref(client, -1);
+	/*
+	 * If there is no pjsip publishing client then we obviously don't need
+	 * to destroy it. Also, the ref for the Asterisk publishing client that
+	 * pjsip had would not exist or should already be gone as well.
+	 */
+	if (client->client) {
+		pjsip_publishc_destroy(client->client);
+		ao2_ref(client, -1);
+	}
 
 	return 0;
 }