Fix segfault in dynamic bridge support

Signed-off-by: Chris Elston <chris.elston@sancloud.com>

Sending an empty packet to $BRIDGE/new would cause a segfault because
the payload pointer was accessed without checking for NULL.

This change rejects an empty payload with MOSQ_ERR_INVAL.

src/bridge_dynamic.c

@@ -418,6 +418,8 @@ int bridge__dynamic_parse_payload_new(struct mosquitto_db *db, void* payload, st
 	int len;
 	int nb_param = 0;
 
+	if(!payload) return MOSQ_ERR_INVAL;
+
 	buf = strtok(payload, "\n");
 
 	while(buf) {