fs/squashfs: simplify sqfs_read()

* Don't check argument of free(). Free does this itself.
* Reduce scope of data_buffer. Remove duplicate free().
* Avoid superfluous NULL assignment.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>