mtd: ofpart: Fix incorrect NULL check in parse_ofoldpart_partitions()

The pointer returned by kzalloc should be tested for NULL
to avoid potential NULL pointer dereference later. Incorrect
pointer was being tested for NULL. Bug introduced by commit fbcf62a3
(mtd: physmap_of: move parse_obsolete_partitions to become separate
parser).
This patch fixes this bug.

Cc: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Cc: Artem Bityutskiy <artem.bityutskiy@intel.com>
Cc: stable@kernel.org
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>