gluploadelement: Avoid race condition of inside upload creation.
The operations for the inside GstGLUploadElement->upload have race condition. The _transform_caps() will creates this object if it does not exist, while the _stop() and change_state() can destroy this object. The _transform_caps() is called by the gst_base_transform_query(), so it does not hold the stream lock. It may use the upload while the _stop() and change_state() has already destroy that object, and then crash. Fix: #645 Part-of: <gstreamer/gst-plugins-base!913>
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