smb3: missing null check in SMB2_change_notify

If plen is null when passed in, we only checked for null
in one of the two places where it could be used. Although
plen is always valid (not null) for current callers of the
SMB2_change_notify function, this change makes it more consistent.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Closes: https://lore.kernel.org/all/202305251831.3V1gbbFs-lkp@intel.com/
Signed-off-by: Steve French <stfrench@microsoft.com>