Fix use-after-free in some cases of reassigning a table index.

Specifically observed when redef'ing the same index of a table that uses
subnets as indices, though the bug seems like it applies more generally
to anytime TableVal::Assign is provided with just the HashKey parameter
and not the index Val.

Addresses BIT-1202.
Merge remote-tracking branch 'origin/fastpath'

* origin/fastpath:
  Fix use-after-free in some cases of reassigning a table index.