Fix coredump when starting up lumina-fm

[maxsteciuk]

It seems the issue was caused by some corrupted QImage pointer during its dereferencing in order to pass to futureFinished() slot

Changing slot signature from call-by-value to call-by-reference / pointer and adding explicit check for std::nullptr fixed the crash problem. The lumina-fm started up successfully.

Here is one GDB backtrace

Core was generated by `lumina-fm'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0 0x0000000800d18006 in QImage::QImage(QImage const&) () from /usr/local/lib/qt5/libQt5Gui.so.5
[Current thread is 1 (LWP 102634)]
(gdb) bt
#0 0x0000000800d18006 in QImage::QImage(QImage const&) () at /usr/local/lib/qt5/libQt5Gui.so.5
#1 0x00000000003266f7 in ()
#2 0x0000000801651641 in QObject::event(QEvent*) () at /usr/local/lib/qt5/libQt5Core.so.5
#3 0x000000080067af61 in QApplicationPrivate::notify_helper(QObject*, QEvent*) () at /usr/local/lib/qt5/libQt5Widgets.so.5
#4 0x000000080067c359 in QApplication::notify(QObject*, QEvent*) () at /usr/local/lib/qt5/libQt5Widgets.so.5
#5 0x0000000801626ee1 in QCoreApplication::notifyInternal2(QObject*, QEvent*) () at /usr/local/lib/qt5/libQt5Core.so.5
#6 0x0000000801627da9 in QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) () at /usr/local/lib/qt5/libQt5Core.so.5
#7 0x0000000801678fd8 in QEventDispatcherUNIX::processEvents(QFlagsQEventLoop::ProcessEventsFlag) () at /usr/local/lib/qt5/libQt5Core.so.5
#8 0x0000000801622a2e in QEventLoop::exec(QFlagsQEventLoop::ProcessEventsFlag) () at /usr/local/lib/qt5/libQt5Core.so.5
#9 0x000000080145fdec in QThread::exec() () at /usr/local/lib/qt5/libQt5Core.so.5
#10 0x0000000801460c83 in () at /usr/local/lib/qt5/libQt5Core.so.5
#11 0x00000008018d5756 in thread_start (curthread=0x8061a5100) at /usr/src_tmp/lib/libthr/thread/thr_create.c:291
#12 0x0000000000000000 in ()

[beanpole135]

Looks good to me!