Don't dereference params->start unconditionally.

Change-Id: I92d0867434a7668a81278de82af45d0a29b9d1e6
Reviewed-on: https://code-review.googlesource.com/c/re2/+/56216


Reviewed-by: Paul Wankadia <junyer@google.com>

re2/dfa.cc

@@ -1702,6 +1702,7 @@ bool DFA::AnalyzeSearch(SearchParams* params) {
   // different state!
   if (prog_->first_byte() >= 0 &&
       !params->anchored &&
+      params->start > SpecialStateMax &&
       params->start->flag_ >> kFlagNeedShift == 0)
     params->have_first_byte = true;