fixup! stbt.py: Fix teardown hang with BlackMagic Intensity Pro

Fix a race in the recently modified teardown code.  With further testing
with the Teradek VidiU I got the error:

    stbt-run: teardown: Source pipeline did not teardown gracefully
    Traceback (most recent call last):

      File "stbt.py", line 1408, in teardown
        self.source_pipeline.set_state(Gst.State.NULL)
    AttributeError: 'NoneType' object has no attribute 'set_state'

As `rtmpsrc` has its own teardown problems (see #111).