Reject invalid fragmented payloads (#257)

* Reject invalid fragmented payloads

- On Arduino, there is a need to reject invalid fragmented payloads with less than 2 fragments

* Slight modification

- header->reserved is used for header->type on the last fragment. Allow low numbers for the last fragment

Author: TMRh20

RF24Network.cpp

@@ -423,6 +423,10 @@ uint8_t RF24Network::enqueue(RF24NetworkHeader* header)
 
     if (isFragment) {
 
+        if (header->reserved < 2 && header->type != NETWORK_LAST_FRAGMENT) {
+            return false;
+        }
+
         if (header->type == NETWORK_FIRST_FRAGMENT) {
 
             memcpy((char*)(&frag_queue), &frame_buffer, sizeof(RF24NetworkHeader));