fs-udf: Delete an unnecessary check before brelse()

The brelse() function tests whether its argument is NULL
and then returns immediately.
Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Link: https://lore.kernel.org/r/a254c1d1-0109-ab51-c67a-edc5c1c4b4cd@web.de


Signed-off-by: Jan Kara <jack@suse.cz>