malloc: fix out-of-bounds segment array access

malloc: fix out-of-bounds segment array access

Technically, while the pointer would've been invalid if msl_idx
were invalid, we wouldn't have actually attempted to access the
pointer until verifying the index. Fix it by moving array access
to after we've verified validity of the index.

Coverity issue: 272574
Fixes: 66cc45e293ed ("mem: replace memseg with memseg lists")
Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Harry van Haaren <harry.van.haaren@intel.com>