mtd: sst25l: check for null consistently

The rest of the function assumes that "data" can be null.  I don't know
the code well enough to say whether it can actually be null, but there
is no harm in checking here.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>