QOpenGLTextureBlitterPrivate::blit crashes due to unexpected null context (VxWorks)

While testing Qt on VxWorks on the i.MX6 board, we noticed a crash in QOpenGLTextureBlitterPrivate::blit.

QBlitterTextureBinder in constructor uses QOpenGLContext::currentContext(), it will crash if currentContext() is nullptr.

Solution is to return from QOpenGLTextureBlitterPrivate::blit before creating QBlitterTextureBinder, when currentContext() is not set.