smb: client: remove redundant NULL check before kfree()

kfree() safely handles NULL pointers, so the explicit NULL check
before calling kfree() is unnecessary.

This issue was reported by ifnullfree.cocci.

Signed-off-by: Mohammad Shahid <mdshahid03@gmail.com>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>