Fix AsyncUDP receive memory leak (#2607)

If _handler is set, pbuf_free is not called. ~AsyncUDPPacket() calls pbuf_free once but only after calling pbuf_ref in it's constructor. The refcount never reaches zero and the memory allocated for pbuf is never released.

libraries/AsyncUDP/src/AsyncUDP.cpp

@@ -682,9 +682,8 @@ void AsyncUDP::_recv(udp_pcb *upcb, pbuf *pb, const ip_addr_t *addr, uint16_t po
         if(_handler) {
             AsyncUDPPacket packet(this, this_pb, addr, port, netif);
             _handler(packet);
-        } else {
-            pbuf_free(this_pb);
         }
+        pbuf_free(this_pb);
     }
 }