examples/filtering_audio,video: drop call to avcodec_get_frame_defaults()

The avcodec_get_frame_defaults() function is deprecated and its use
doesn't seem required.
lavf/mpegtsenc: fix weird indent
lavf/segment: remove duplicated and inconsistent cleanup code in seg_write_packet()

In particular, avoid to leave around the seg->avf pointer to freed
structure, and fix crash with:
ffmpeg -f lavfi -i testsrc -c:v h264 -map 0 -f segment foo-%d.ts
lavf/segment: drop pointless variable oc from seg_write_packet()