[tftp] Guard against invalid data block numbers

A TFTP DATA packet with a block number of zero (representing a
negative offset within the file) could potentially cause problems.
Fixed by explicitly rejecting such packets.

Identified by Stefan Hajnoczi <stefanha@gmail.com>.

src/net/udp/tftp.c

 		rc = -EINVAL;
 		goto done;
 	}
+	if ( data->block == 0 ) {
+		DBGC ( tftp, "TFTP %p received data block 0\n", tftp );
+		rc = -EINVAL;
+		goto done;
+	}
 
 	/* Extract data */
 	block = ( ntohs ( data->block ) - 1 );