[tls] Fix uninitialised variable

Reported-by: Christian Hesse <list@eworm.de>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/tls.c

 	}
 
 	/* Handle record and free I/O buffer */
-	if ( handler )
-		rc = handler ( tls, iobuf->data, iob_len ( iobuf ) );
+	rc = ( handler ? handler ( tls, iobuf->data, iob_len ( iobuf ) ) : 0 );
 	free_iob ( iobuf );
 	return rc;
 }