Fix harmless off-by-1 error.

The memory being zeroed in vp8_update_mode_info_border() was just
 allocated with calloc, and so the entire function is actually
 redundant, but it should be made correct in case someone expects
 it to actually work in the future.

Change-Id: If7a84e489157ab34ab77ec6e2fe034fb71cf8c79