rabbitmq: proper test of pv cache lookup return

(cherry picked from commit de4a32b02ddaf94b0ad4ed06a0b4fa5bb9c1a10e)

src/modules/rabbitmq/rabbitmq.c

@@ -507,7 +507,7 @@ static int ki_rabbitmq_publish_consume(sip_msg_t *msg, str *exchange,
 
 	dst = pv_cache_get(dpv);
 
-	if(dpv==NULL) {
+	if(dst==NULL) {
 		LM_ERR("failed getting pv: %.*s\n", dpv->len, dpv->s);
 		return -1;
 	}
@@ -669,4 +669,4 @@ int mod_register(char *path, int *dlflags, void *p1, void *p2)
 {
 	sr_kemi_modules_add(sr_kemi_rabbitmq_exports);
 	return 0;
-}
+}