ip_mroute: handle V_mfchashtbl allocation failure

We allocate V_mfchashtbl with HASH_NOWAIT (which maps to M_NOWAIT), so
this allocation may fail. As we didn't handle that failure we could end
up dereferencing a NULL pointer later (e.g. during X_ip_mrouter_done()).

Do the obvious thing and fail out if we cannot allocate the table.

See also: https://redmine.pfsense.org/issues/14917
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