avformat/matroskaenc: check packet side data for AAC extradata updates

This adapts and merges commit f4bf236338f6001736a4784b9c23de863057a583
from libav, originally skipped in 13a211e6320d061d9e8c29354c81239324b2db03
as it was not necessary back then.

Is's applied now in preparation for the following patches, where the
aac_adtstoasc bitstream filter will start to correctly propagate the new
extradata through packet side data.

Signed-off-by: James Almer <jamrial@gmail.com>
