Paul, Thank you always for everything.  Fix for dfaisfast() is right.  A
superset never has BACKREF.  However, I think that we don't have to check
the end pointer when dfaisfast is true, because run DFA for whole a buffer
(use buflim) and check the end pointer.

By the way, I took into another bug by my previous patch.  If
`kwsm.index < kwset_exact_matches', don't have to run DFA for whole a buffer.

Norihiro