gstreamer: Fix leak using GstBus watch

This patch fixes a leak due to not freeing GstBus watch.
The watch is attached (as GSource) to the main loop and retains
a pointer to the bus so we need to remove it to release the bus
when we release the pipeline.
This was detected forcibly creating and destroying lot of streams.
After a while the client program consumed all file descriptors
and stopped working. This as GstBus retains a GPoll which,
under Unix, uses 2 file descriptors.
For some reasons using gst_pipeline_get_bus again in free_pipeline
do not fix entirely the leak so keep a pointer to the exact
bus we set our watch on.

Signed-off-by: Frediano Ziglio <freddy77@gmail.com>
Acked-by: Vivek Kasireddy <vivek.kasireddy@intel.com>