webm2pes: avoid null deref w/track->GetCodecId()
webm2pes: avoid null deref w/track->GetCodecId()

check pointer value before converting to string

fixes CVE-2018-19212

BUG=webm:1605

Change-Id: Idbe51dc8e0e454019003ba1ac7afa175f7bf8950

webmproject/libwebm,webmproject/libwebm,webmproject/libwebm