dfa: disable if size limit is 0
dfa: disable if size limit is 0

As a special case, if the user configures a DFA size limit of 0, then we
should never try to use it. This avoids a bit of thrashing where the DFA
tries to senselessly run before spilling over to the NFA.

rust-lang-nursery/regex,rust-lang-nursery/regex,rust-lang-nursery/regex,rust-lang-nursery/regex,rust-lang/regex,rust-lang/regex,rust-lang-nursery/regex,rust-lang/regex,rust-lang/regex,rust-lang/regex