Crash in sprite viewer

[07151129]

mGBA consistently crashes on OSX with the following backtrace when trying to examine a pixel from a tile in the sprite viewer:

Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0   libmgba.0.8.dylib             	0x00000001086fac0b mTileCacheGetTile + 267
1   com.endrift.mgba-qt           	0x000000010833697d QGBA::AssetTile::selectColor(int) + 61
2   org.qt-project.QtCore         	0x000000010b2d998b QMetaObject::activate(QObject*, int, int, void**) + 1907
3   com.endrift.mgba-qt           	0x00000001083ed62c QGBA::Swatch::mousePressEvent(QMouseEvent*) + 220
4   org.qt-project.QtWidgets      	0x0000000108f8eef3 QWidget::event(QEvent*) + 1211
5   org.qt-project.QtWidgets      	0x0000000108f60d33 QApplicationPrivate::notify_helper(QObject*, QEvent*) + 269
6   org.qt-project.QtWidgets      	0x0000000108f637b9 QApplication::notify(QObject*, QEvent*) + 6765
7   org.qt-project.QtCore         	0x000000010b2b44c4 QCoreApplication::notifyInternal2(QObject*, QEvent*) + 154
8   org.qt-project.QtWidgets      	0x0000000108f615b7 QApplicationPrivate::sendMouseEvent(QWidget*, QMouseEvent*, QWidget*, QWidget*, QWidget**, QPointer<QWidget>&, bool) + 897
9   org.qt-project.QtWidgets      	0x0000000108fa7075 0x108f53000 + 344181
10  org.qt-project.QtWidgets      	0x0000000108fa5d3e 0x108f53000 + 339262
11  org.qt-project.QtWidgets      	0x0000000108f60d33 QApplicationPrivate::notify_helper(QObject*, QEvent*) + 269
12  org.qt-project.QtWidgets      	0x0000000108f62018 QApplication::notify(QObject*, QEvent*) + 716
13  org.qt-project.QtCore         	0x000000010b2b44c4 QCoreApplication::notifyInternal2(QObject*, QEvent*) + 154
14  org.qt-project.QtGui          	0x00000001094577b3 QGuiApplicationPrivate::processMouseEvent(QWindowSystemInterfacePrivate::MouseEvent*) + 2615
15  org.qt-project.QtGui          	0x00000001094466c5 QWindowSystemInterface::sendWindowSystemEvents(QFlags<QEventLoop::ProcessEventsFlag>) + 149
16  libqcocoa.dylib               	0x00000001107095da 0x1106e3000 + 157146
17  libqcocoa.dylib               	0x0000000110709d33 0x1106e3000 + 159027
18  com.apple.CoreFoundation      	0x00007fff40415475 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
19  com.apple.CoreFoundation      	0x00007fff4041541b __CFRunLoopDoSource0 + 108
20  com.apple.CoreFoundation      	0x00007fff403f8eb5 __CFRunLoopDoSources0 + 195
21  com.apple.CoreFoundation      	0x00007fff403f845e __CFRunLoopRun + 1219
22  com.apple.CoreFoundation      	0x00007fff403f7d48 CFRunLoopRunSpecific + 463
23  com.apple.HIToolbox           	0x00007fff3f68eab5 RunCurrentEventLoopInMode + 293
24  com.apple.HIToolbox           	0x00007fff3f68e6f4 ReceiveNextEventCommon + 371
25  com.apple.HIToolbox           	0x00007fff3f68e568 _BlockUntilNextEventMatchingListInModeWithFilter + 64
26  com.apple.AppKit              	0x00007fff3d949363 _DPSNextEvent + 997
27  com.apple.AppKit              	0x00007fff3d948102 -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 1362
28  com.apple.AppKit              	0x00007fff3d942165 -[NSApplication run] + 699
29  libqcocoa.dylib               	0x0000000110708dd1 0x1106e3000 + 155089
30  org.qt-project.QtCore         	0x000000010b2b0fe0 QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) + 318
31  org.qt-project.QtCore         	0x000000010b2b4978 QCoreApplication::exec() + 116
32  com.endrift.mgba-qt           	0x0000000108332a2a main + 4874
33  libdyld.dylib                 	0x00007fff6d639ed9 start + 1

[endrift]

Good catch! Fixed now.