avformat/movenc: Fix segfault when remuxing rtp hint stream

avformat/movenc: Fix segfault when remuxing rtp hint stream

When remuxing an rtp hint stream (or any stream with the tag "rtp "),
the mov muxer treats this as one of the rtp hint tracks it creates
internally when ordered to do so; yet this track lacks the
AVFormatContext for the hinting rtp muxer, leading to segfaults in
mov_write_udta_sdp() if a "trak" atom is written for this stream; if not,
the stream's codecpar is freed by mov_free() as if the mov muxer owned
it (it does for the internally created "rtp " tracks), but without
resetting st->codecpar, leading to double-frees lateron. This commit
therefore ignores said tag which makes rtp hint streams unremuxable.

This fixes tickets #8181 and #8186.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>