beginPacket can be used without listening on socket (#185)

Currently there is bug in WiFiUDP library when you want to use beginPacket(...) without listening on socket (without calling begin(...) first). You can't send any data because socket is not open and also tx_buffer is not allocated which cause crash while writing data to tx_buffer.

libraries/WiFi/src/WiFiUdp.cpp

@@ -130,7 +130,29 @@ int WiFiUDP::beginMulticastPacket(){
 int WiFiUDP::beginPacket(){
   if(!remote_port)
     return 0;
+
+  // allocate tx_buffer if is necessary
+  if(!tx_buffer){
+    tx_buffer = new char[1460];
+    if(!tx_buffer){
+      log_e("could not create tx buffer: %d", errno);
+      return 0;
+    }
+  }
+
   tx_buffer_len = 0;
+
+  // check whereas socket is already open
+  if (udp_server != -1)
+    return 1;
+
+  if ((udp_server=socket(AF_INET, SOCK_DGRAM, 0)) == -1){
+    log_e("could not create socket: %d", errno);
+    return 0;
+  }
+
+  fcntl(udp_server, F_SETFL, O_NONBLOCK);
+
   return 1;
 }