h264parse: don't invalidate the last PPS when parsing a new SPS

When a SPS is received then any previous PPS remains valid. So don't clear
the PPS flag from the parser state.

This is important because there are encoders that don't generated a PPS after
every SPS.

Closes gstreamer/gst-plugins-bad#571

Part-of: <gstreamer/gst-plugins-bad!2243>
29 jobs for !2243 with backport-mr-2019-into-1.18-h264parse in 38 minutes and 25 seconds (queued for 2 seconds)
merge request