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.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: stable@vger.kernel.org [3.2+]
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>