windowscodecs: Add a NULL check to GetFrameCount of the JPEG decoder.

dlls/windowscodecs/jpegformat.c

@@ -430,6 +430,8 @@ static HRESULT WINAPI JpegDecoder_GetThumbnail(IWICBitmapDecoder *iface,
 static HRESULT WINAPI JpegDecoder_GetFrameCount(IWICBitmapDecoder *iface,
     UINT *pCount)
 {
+    if (!pCount) return E_INVALIDARG;
+
     *pCount = 1;
     return S_OK;
 }