handle race in RegexMatchCache is-ready and prepare sequence

Summary:
The race is when two threads both do:
```
// hold shared lock
if (!cache.isReadyToFindMatches(key)) {
  // drop shared lock
  // acquire exclusive lock
  cache.prepareToFindMatches(key);
  // transition exclusive to shared lock
}
```

Differential Revision: D64149453

fbshipit-source-id: b8ed1c85a4d73f259c0b7aa4c2b5ed82d8474d6c

folly/container/RegexMatchCache.cpp

@@ -448,7 +448,11 @@ void RegexMatchCache::prepareToFindMatches(regex_key_and_view const& regex) {
   } else {
     //  evaluate old regex over queue
     auto const sqriter = stringQueueReverse_.find(regexp);
-    CHECK(sqriter != stringQueueReverse_.end());
+    if (sqriter == stringQueueReverse_.end()) {
+      //  was actually ready-to-find-matches for regex
+      guard.dismiss();
+      return;
+    }
     auto const strings = std::move(sqriter->second.strings);
     CHECK(!strings.empty());
     stringQueueReverse_.erase(sqriter);