gluploadelement: segfault in the middle of stateChange PAUSED to NULL

When setting the state of the pipeline to NULL in the middle of changing its state to PAUSED, before get PAUSED, it could lead to misleading behavior of gluploadelement which re-create glupload in it. It is related to this change. acd33a24 It could be a problem to create glupload in the transform_caps.

It can be rarely reproduced with videotestsrc ! glimagesink pipeline.