Fix a potential NULL dereference in md/raid1

At the point where this 'atomic_add' is, rdev could be NULL,
as seen by the fact that we test for this in the very next
statement.
Further is it is really the wrong place of the add.
We could add to the count of corrected errors
once the are sure it was corrected, not before
trying to correct it.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Adrian Bunk <bunk@stusta.de>