avformat/webmdashenc: fix uninitialized variable

Fixes -Wsometimes-uninitialized from
http://fate.ffmpeg.org/report.cgi?time=20150820031140&slot=arm64-darwin-clang-apple-5.1

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Reviewed-by: "Ronald S. Bultje" <rsbultje@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
