matroskaenc: fix muxing AAC streams when using aac_adtstoasc bsf

matroskaenc: fix muxing AAC streams when using aac_adtstoasc bsf

aac_adtstoasc makes the aac extradata available only after the first packet
is filtered, and as packet side data.

Assume extradata will be available as part of the first packet if
avpriv_mpeg4audio_get_config() fails the first time due to missing extradata
and reserve space for the OutputSampleRate element in the Tracks master.

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>