When a new payload type is encountered, we first check whether
it matches the caps received as an event before emitting the
request-pt-map signal if not, which means we shouldn't consider
errors from the first call to parse_caps as fatal.
Not returning NoPreroll in that transition causes downstream sinks
to wait for preroll forever.
Reproduce with:
gst-launch-1.0 audiotestsrc ! udpsink host=127.0.0.1 port=50000
gst-launch-1.0 ts-udpsrc address=127.0.0.1 port=50000 ! fakesink
ctrl + C in the receiver pipeline -> hangs forever