mkvparser/SeekHead: convert asserts to failure returns
mkvparser/SeekHead: convert asserts to failure returns

Change-Id: Ic2c83e4eb246fd4c1fc63fe16298f0dc78801912

webmproject/libwebm,felipebetancur/libwebm,webmproject/libwebm,zofuthan/libwebm,webmproject/libwebm,felipebetancur/libwebm,zofuthan/libwebm