[qt] Fix crash on Qt filesource

When opening many maps and sharing the same filesource:

- Say we open around 30 maps and they all have the same style.
- They will all request the same sprite.json, but Qt filesource will
  restrict simultaneous request to 20.
- Mapbox GL will send some sprite.json to the filesource and queue
  some internally along with other requests.
- When the first sprite.json arrives and the Qt filesource calls the
  callback, Mapbox GL will synchronously push queued requests to the
  Qt filesource when a request is handled.
- We are walking a QVector dispatching sprite.json requests, but more
  requests get added to this QVector while we are walking it using an
  iterator.
- That corrupts the iterator and we get a crash.

The fix is simple, just pop items from the QVector until it is empty
instead of using iterators.

platform/qt/src/http_file_source.cpp

@@ -75,8 +75,12 @@ void HTTPFileSource::Impl::onReplyFinished()
 
     QByteArray data = reply->readAll();
     QVector<HTTPRequest*>& requestsVector = it.value().second;
-    for (auto req : requestsVector) {
-        req->handleNetworkReply(reply, data);
+
+    // Cannot use the iterator to walk the requestsVector
+    // because calling handleNetworkReply() might get
+    // requests added to the requestsVector.
+    while (!requestsVector.isEmpty()) {
+        requestsVector.takeFirst()->handleNetworkReply(reply, data);
     }
 
     m_pending.erase(it);