Set pipeline to NULL after terminate signal

[mhkabir]

^C[gst_streamer-1] killing on exit

(server_node:31914): GStreamer-CRITICAL **: 
Trying to dispose element source, but it is in PLAYING instead of the NULL state.
You need to explicitly set elements to the NULL state before
dropping the final reference, to allow them to clean up.
This problem may also be caused by a refcounting bug in the
application or some element.

[vooon]

Yes, that message i got too. But i'm not sure why. It should be released in class deinit.

[vooon]

But for early development that issue has too low importance.