ip: reassembly: fix one possible use-after-free

ip: reassembly: fix one possible use-after-free

When use the kv->v.memory_owner_thread_index as the index to get the
reass in pool, maybe this element is freed by the owner thread because
of timeout, too many fragments, and so on.

So we should check if do_handoff with kv->v.memory_owner_thread_index
before get the reass from pool.

Type: fix

Signed-off-by: Gao Feng <davidfgao@tencent.com>
Change-Id: Ie0f1dc368f86d0fd65292ca0c5e1908348015e09