BUG/MAJOR: counters: check for null-deref when looking up an alternate table

Constructions such as sc0_get_gpc0(foo) allow to look up the same key as
the current key but in an alternate table. A check was missing to ensure
we already have a key, resulting in a crash if this lookup is performed
before the associated track-sc rule.

This bug was reported on the mailing list by Neil@iamafreeman and
narrowed down further by Lukas Tribus and Thierry Fournier.

This bug was introduced in 1.5-dev20 by commit "0f791d4 MEDIUM: counters:
support looking up a key in an alternate table".