add verification data ptr not null befor copy, user data can be null with somme tcp frames

library/driver/ethernet/patmos_eth_mac.c

@@ -356,8 +356,12 @@ network_error_t eth_mac_send_nb(network_frame_t *frame, mmio_ptr_t config_ptr)
     // adapt address and get next frame segment
     address += frame->header_length;
     
-    // write user data into buffer
-    mmio_wr_block(address, frame->user_data, frame->user_data_length);    
+    // user data can be null sometimes with TCP frames
+    if(frame->user_data != NULL)
+    {
+        // write user data into buffer
+        mmio_wr_block(address, frame->user_data, frame->user_data_length);    
+    }
     
     unsigned intr_en = EXC_STATUS & 0x1;
     if(intr_en)

library/driver/ethernet/patmos_tsn_mac.c

@@ -642,8 +642,12 @@ network_error_t tsn_mac_send_nb(network_frame_t *frame, mmio_ptr_t config_ptr)
     // adapt address and get next frame segment
     address += frame->header_length;
     
-    // write user data into buffer
-    mmio_wr_block(address, frame->user_data, frame->user_data_length);    
+    // user data can be null sometimes with TCP frames
+    if(frame->user_data != NULL)
+    {
+        // write user data into buffer
+        mmio_wr_block(address, frame->user_data, frame->user_data_length);    
+    }
 
     // enques the frame 
     time_stamp_t dummy_ts;