avformat/matroskaenc: Cosmetics

Reindentation, removal of { } if they contain only one statement
and moving the return statement to a line of its own in situations
like "if (ret < 0) return ret;". Moreover, several overlong lines
were made shorter and a camelCase variable received a name in line
with our naming conventions.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>