fs: avoid possible NULL dereference in fs_devread

It is unwise to first dereference a variable
and then to check if it was NULL.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Marek Behun <marek.behun@nic.cz>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>