Free extradata before reallocating.

Otherwise ff_alloc_extradata() just leaks any existing allocated
memory.
Signed-off-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/oggparseogm.c

@@ -110,6 +110,7 @@ ogm_header(AVFormatContext *s, int idx)
                 size -= 52;
                 if (bytestream2_get_bytes_left(&p) < size)
                     return AVERROR_INVALIDDATA;
+                av_freep(&st->codecpar->extradata);
                 if (ff_alloc_extradata(st->codecpar, size) < 0)
                     return AVERROR(ENOMEM);
                 bytestream2_get_buffer(&p, st->codecpar->extradata, st->codecpar->extradata_size);