avformat/utils: Fix find_stream_info not considering the extradata it found

Commit 9200514ad8717c6 ("lavf: replace AVStream.codec with
AVStream.codecpar") merged in commit 6f69f7a8bf6a0d01 changed
avformat_find_stream_info() to put the extradata it got from
st->parser->parser->split() to st->internal->avctx instead of st->codec
(extradata in st->internal->avctx will be later copied to st->codecpar).

However, in the same function, the "is stream ready?" check was changed
to check for extradata in st->codecpar instead of st->codec, even
though st->codecpar is not yet updated at that point.

Extradata retrieved from split() is therefore not considered anymore,
and avformat_find_stream_info() will therefore needlessly continue
probing in some cases.

Fix that by checking for the extradata at st->internal->avctx where it
is actually put.
