gstreamer: Avoid dangling pointers in free_pipeline

Although currently after free_pipeline we free the entire structure
the name and the function suggests that we only free the pipeline.
Also this is fixing a future possible problem with the series
from Vivek Kasireddy reusing the SpiceGstDecoder for another
pipeline if H/W encoder pipeline creation fails.

Signed-off-by: Frediano Ziglio <freddy77@gmail.com>