qt_imageToWinHBITMAP(): properly delete object on error

Properly delete the handle created by CreateDIBSection() with
DeleteObject() in case of error.

Pick-to: 6.6 6.5
Fixes: QTBUG-18057
Change-Id: Ibf40f6afb255bc5a7d7bf39939f5eb3dfcd19f84
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
(cherry picked from commit b37b8eb7ea0a809c6e96189e8f9e834d6763e170)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/image/qpixmap_win.cpp

@@ -305,6 +305,7 @@ HBITMAP qt_imageToWinHBITMAP(const QImage &imageIn, int hbitmapFormat)
         return nullptr;
     }
     if (!pixels) {
+        DeleteObject(bitmap);
         qErrnoWarning("%s, did not allocate pixel data", __FUNCTION__);
         return nullptr;
     }