pixmap: Add QPlatformPixmap::fromData null check for the image

QPlatformPixmap::fromFile is doing the null check,
QRasterPlatformPixmap::fromData is doing the null check as
well. Add the null check to QPlatformPixmap::fromData as
well. This would have avoided a crash in the directfb plugin.

Change-Id: Ifc904b2df2424e86bf0992d4e28dc256530be02d
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>

src/gui/image/qplatformpixmap.cpp

@@ -133,6 +133,8 @@ bool QPlatformPixmap::fromData(const uchar *buf, uint len, const char *format, Q
     QBuffer b(&a);
     b.open(QIODevice::ReadOnly);
     QImage image = QImageReader(&b, format).read();
+    if (image.isNull())
+        return false;
     fromImage(makeBitmapCompliantIfNeeded(this, image, flags), flags);
     return !isNull();
 }