Always initialize values in sb/mb_partitioning.

This doesn't affect the output, because in previous cases where the
values were uninitialized, this was because the mb_row/col is outside
the codable area, and thus encode_sb will test them for the next
decomposition-level, but return right after that on size-check. All
this does is prevent a warning in valgrind.

Change-Id: I90d8a29e6f8ebb2b0143684e08fe77ae3a0816b1