malloc: check for heap corruption

malloc: check for heap corruption

Previous code checked for both first/last elements being NULL,
but if they weren't, the expectation was that they're both
non-NULL, which will be the case under normal conditions, but
may not be the case due to heap structure corruption.

Coverity issue: 272566
Fixes: bb372060dad4 ("malloc: make heap a doubly-linked list")
Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Harry van Haaren <harry.van.haaren@intel.com>