Fix race condition between WiFi receive and consume (#1614)

If another packet comes in between freeing `_rx_buf` and setting `_rx_buf` to 0, that new packet could get put into the same memory address and get concatenated to itself, which leads to an infinite loop.
New solution assigns a temp pointer, sets `rx_buf` to 0, then frees the memory, which guarantees `_rx_buf` always points to valid data.

libraries/WiFi/src/include/ClientContext.h

@@ -611,9 +611,10 @@ protected:
             _rx_buf_offset += size;
         } else if (!_rx_buf->next) {
             DEBUGV(":c0 %d, %d\r\n", size, _rx_buf->tot_len);
-            pbuf_free(_rx_buf);
+            auto head = _rx_buf;
             _rx_buf = 0;
             _rx_buf_offset = 0;
+            pbuf_free(head);
         } else {
             DEBUGV(":c %d, %d, %d\r\n", size, _rx_buf->len, _rx_buf->tot_len);
             auto head = _rx_buf;