gst/rtp/: Use is_filled to both check MTU and max-ptime of base class.

gst/rtp/: Use is_filled to both check MTU and max-ptime of base class.

Original commit message from CVS:
* gst/rtp/TODO:
* gst/rtp/gstrtpdec.c: (gst_rtpdec_getcaps):
* gst/rtp/gstrtpmp4venc.c: (gst_rtpmp4venc_class_init),
(gst_rtpmp4venc_parse_data), (gst_rtpmp4venc_handle_buffer),
(gst_rtpmp4venc_set_property):
* gst/rtp/gstrtpmp4venc.h:
* gst/rtp/gstrtpmpaenc.c: (gst_rtpmpaenc_handle_buffer):
* gst/rtp/gstrtpmpaenc.h:
Use is_filled to both check MTU and max-ptime of base class.