Don't try to look in a non-existent hash table.

We only create hash tables if somebody puts a handler in one, so we need
to check whether the hash table exists first, to avoid run-time warnings.

Change-Id: I739d2d808935e651b11bd44b258f168a42ca4b7c
Reviewed-on: https://code.wireshark.org/review/5683
Reviewed-by: Guy Harris <guy@alum.mit.edu>