BUG/MAJOR: stick-counters: possible crash when using sc_trackers with wrong table

BUG/MAJOR: stick-counters: possible crash when using sc_trackers with wrong table

Bryan Talbot reported a very interesting bug. The sc_trackers() sample
fetch seems to have escaped the sanitization that was performed during 1.5
to ensure all dereferences of stkctr_entry() were safe. Here if a tacker
is set on a backend and is then checked against a different backend where
the entry doesn't exist, stkctr_entry() returns NULL and this is dereferenced
to retrieve the ref count. Thanks to Bryan for his detailed bug report featuring
a working config and reproducer.

This fix must be backported to 1.6 and 1.5.