In multi-thread case, the encoder may crash if using encoder option
tile-rows > 0. To prevent that, force tile-rows=0 in this situation.
This is a workaround for WebM issue 1095:
https://bugs.chromium.org/p/webm/issues/detail?id=1095

The further fix can be done by adding synchronizations after a tile
row is encoded. But this will hurt multi-threaded encoder performance.
So, it is recommended to use tile-rows=0 while encoding with threads
> 1.

Change-Id: I656cbcc200f8d0410d09530e7981ad8f32fe7bc9