Bugfix in order to make sr_session_stop() actually work.

Bugfix in order to make sr_session_stop() actually work.

The sr_session_stop() calls were previously invoked with the session
pointer being NULL, which didn't actually stop the session. This could
lead to samples still being processed and (e.g.) passed to libsigrokdecode
for decoding, even though the session should have been stopped.

Fix this by passing the session pointer along in the
sr_session_datafeed_callback_add() function, so that the callbacks
can make use of it.