Fixes crash in gif image decoder

Fuzzing test revealed that for certain malformed gif files,
qgifhandler would segfault.

Change-Id: I5bb6f60e1c61849e0d8c735edc3869945e5331c1
Reviewed-by: Richard J. Moore <rich@kde.org>

src/gui/image/qgifhandler.cpp

@@ -936,6 +936,8 @@ void QGIFFormat::fillRect(QImage *image, int col, int row, int w, int h, QRgb co
 
 void QGIFFormat::nextY(unsigned char *bits, int bpl)
 {
+    if (out_of_bounds)
+        return;
     int my;
     switch (interlace) {
     case 0: // Non-interlaced