When writing a subtitle SSA/ASS subtitle file, the AVCodecParameters::extradata
buffer is written directly to the output.  In the case where the buffer is
filled from a matroska source file produced by some older versions of
Handbrake, this buffer ends with a null terminating character, which is then
erroneously copied into the middle of the output file. The change here avoids
this problem by treating it as a string rather than a raw buffer. This way it
is agnostic as to whether the source buffer was null terminated or not.

Fixes ticket #10203.

Reported-by: Tim Angus <tim at ngus.net>
Signed-off-by: Marton Balint <cus@passwd.hu>