-Werror,-Wsign-compare (clang-cl)

According to MSDN: "If [StretchDIBits] fails, or no scan lines are copied, the
return value is 0."

Change-Id: I882b97b55f92618f0163f8829bf779a35b5a1452

fpicker/source/win32/filepicker/dibpreview.cxx

@@ -241,7 +241,7 @@ void SAL_CALL CDIBPreview::onPaint(HWND hWnd, HDC hDC)
                 0, 0, cxDib, cyDib, pBits, pbmi,
                 DIB_RGB_COLORS, SRCCOPY);
 
-            OSL_ASSERT(GDI_ERROR != GDIError);
+            OSL_ASSERT(0 != GDIError);
 
             // paint the border
             RECT rc;