[S390] drivers/s390: put NULL test before dereference

If the NULL test on block is needed, it should be before the dereference of
the base field.

The semantic match that finds this problem is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@r@
expression E1,E2;
identifier fld;
statement S1,S2;
@@

E1 = E2->fld;
(
if (E1 == NULL) S1 else S2
|
*if (E2 == NULL) S1 else S2
)
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>