avformat/webm_chunk: Open AVIOContext before initializing sub-muxer

avformat/webm_chunk: Open AVIOContext before initializing sub-muxer

The description of AVOutputFormat.init contains the statement that "this
method must not write output". Due to this, the webm_chunk muxer defers
opening the AVIOContext for the child muxer until avformat_write_header(),
i.e. there is no AVIOContext when the sub-muxer's avformat_init_output()
is called. But this violates the documentation of said function which
requires the AVFormatContext to have an already opened AVIOContext.
This commit fixes this.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>