avformat/dashenc: Fix leak of AVFormatContext on error
The Dash muxer uses submuxers and when one such submuxer has been allocated,
it is initially only stored in a temporary variable. Therefore it leaks
if an error happens between the allocation and storing it permanently.
This commit changes this.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com> Reviewed-by: "Jeyapal, Karthick" <kjeyapal@akamai.com> Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>