Add null check on the history_prune_re_ to avoid crash.

Bug: 859432
Change-Id: I2fb4b30fffac5db5c2d5a31e7bf96b74e2bcbc89
Reviewed-on: https://chromium-review.googlesource.com/c/1317074Reviewed-by: Leo Zhang <googleo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605509}

chromeos/services/ime/public/cpp/rulebased/rules_data.cc

@@ -335,6 +335,8 @@ bool RulesData::Transform(const std::string& context,
 }
 
 bool RulesData::MatchHistoryPrune(const std::string& str) const {
+  if (!history_prune_re_)
+    return false;
   return re2::RE2::FullMatch(str, *history_prune_re_);
 }