stbt.py: Fix race condition: TypeError on NoVideo

During a NoVideo source pipeline reset `Display.source_pipeline` is `None`.  If simultaneously `get_sample` times out stbt will try to dump the source pipeline graph.  This causes `TypeError` to be raised:

      File ".../stbt/stbt.py", line 1493, in get_sample
        self.source_pipeline, Gst.DebugGraphDetails.ALL, "NoVideo")
    TypeError: Argument 0 does not allow None as a value

Thanks to @lewishaley for reporting this bug and correctly diagnosing the cause.

Fixes drothlis/#197