mkvparser: Avoid alloc failures in SeekHead::Parse.
mkvparser: Avoid alloc failures in SeekHead::Parse.

Some run times return NULL pointers when creating 0 length
arrays. Avoid the inconsistency by skipping allocation when
the length of the SeekHead entry or void element array
is 0.

BUG=webm:1313

Change-Id: I28b4eac1d3e0ba968147637883f6e37c089e8573

webmproject/libwebm,webmproject/libwebm,webmproject/libwebm