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>