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

vp8/common/alloccommon.c

@@ -24,7 +24,7 @@ extern  void vp8_init_scan_order_mask();
 void vp8_update_mode_info_border(MODE_INFO *mi, int rows, int cols)
 {
     int i;
-    vpx_memset(mi - cols - 1, 0, sizeof(MODE_INFO) * cols + 1);
+    vpx_memset(mi - cols - 2, 0, sizeof(MODE_INFO) * (cols + 1));
 
     for (i = 0; i < rows; i++)
     {