After a CAPS event, in theory a new stream can start and it might start
with the FLAC headers again. We can't detect FLAC headers in the middle
of the stream, so we drain the parser to be able to detect either FLAC
headers after the CAPS event or the continuation of the previous stream.

This fixes for example

gst-launch-1.0 audiotestsrc num-buffers=200 ! flacenc ! c. \
    audiotestsrc num-buffers=200 freq=880 ! flacenc ! c. \
    concat name=c ! rtpgstpay ! udpsink host=127.0.0.1 port=5000

gst-launch-1.0 udpsrc multicast-group=127.0.0.1 port=5000 \
    caps=application/x-rtp,media=application,clock-rate=90000,encoding-name=X-GST ! \
    rtpgstdepay ! flacparse ! flacdec ! audioconvert ! pulsesin