Add immediate sanity check on untrusted length

We were actually checking this indirectly in the loop that follows,
but it's more clear to check this immediately and directly.

src/switch_stun.c

@@ -129,7 +129,8 @@ SWITCH_DECLARE(switch_stun_packet_t *) switch_stun_packet_parse(uint8_t *buf, ui
 	packet = (switch_stun_packet_t *) buf;
 	packet->header.type = ntohs(packet->header.type);
 	packet->header.length = ntohs(packet->header.length);
-	bytes_left -= 20;
+	if (packet->header.length > (bytes_left -= 20))
+		return NULL;
 
 
 	/*