When we specifically release all limits on a channel we destroy the
hash table stored in the "limit_hash" private channel data but we
don't destroy the private data as it will be reclaimed as part of the
session.  If limit increment is called after the limit release we can
reuse that channel private, but we need to check whether the hash
table is null first.  Fortunately this makes the code look better
anyway.

FS-6775 #resolve
FS-6783 #resolve