convert to zalloc

src/net/udp/tftp.c

 		return -EINVAL;
 
 	/* Allocate and populate TFTP structure */
-	tftp = malloc ( sizeof ( *tftp ) );
+	tftp = zalloc ( sizeof ( *tftp ) );
 	if ( ! tftp )
 		return -ENOMEM;
-	memset ( tftp, 0, sizeof ( *tftp ) );
 	tftp->refcnt.free = tftp_free;
 	xfer_init ( &tftp->xfer, &tftp_xfer_operations, &tftp->refcnt );
 	tftp->uri = uri_get ( uri );