QQuickPixmapReader::asyncResponseFinished segfaults if a QQuickAsyncImageProvider returns an error response

While working on the new thumbnailer, we've been using the new QQuickAsyncImageProvider API. The API allows us to report errors by overriding the errorString() method on QQuickImageResponse to return a non-empty string. However, if I do so the application crashes.

Loading up the symbols to get a stack trace shows this to be a bug in the logic of QQuickPixmapReader::asyncResponseFinished:

        QQuickTextureFactory *t = 0;
        QQuickPixmapReply::ReadError error = QQuickPixmapReply::NoError;
        QString errorString;
        QSize readSize;
        if (!response->errorString().isEmpty()) {
            error = QQuickPixmapReply::Loading;
            errorString = response->errorString();
        } else {
            t = response->textureFactory();
       }
        mutex.lock();
        if (!cancelled.contains(job))
            job->postReply(error, errorString, t->textureSize(), t);
        mutex.unlock();

If errorString() is not empty, then t will still be NULL. It is then dereferenced to call t->textureSize() resulting in a segfault.

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: libqt5quick5 5.4.2-1ubuntu1
ProcVersionSignature: Ubuntu 3.19.0-20.20-generic 3.19.8
Uname: Linux 3.19.0-20-generic x86_64
ApportVersion: 2.17.3-0ubuntu4
Architecture: amd64
CurrentDesktop: Unity
Date: Mon Jun 29 14:53:49 2015
InstallationDate: Installed on 2013-10-29 (607 days ago)
InstallationMedia: Ubuntu 13.10 "Saucy Salamander" - Release amd64 (20131016.1)
SourcePackage: qtdeclarative-opensource-src
UpgradeStatus: Upgraded to wily on 2015-06-13 (15 days ago)

Test case (on desktop):

- bzr branch lp:~jamesh/thumbnailer/no-fallback-albumart
- sudo apt-get build-dep thumbnailer
- sudo apt install libleveldb-dev cmake-extras libapparmor-dev libboost-filesystem-dev libboost-regex-dev libqtdbustest1-dev libunity-api-dev python3-tornado qml-module-qttest xvfb
- cd no-fallback-albumart
- cmake .
- make
- ctest -R qml --verbose
-> check if crash (Segmentation fault (core dumped)) or no crash (don't mind tests pass/fail)