Moves error concealment allocations from common parts to decoder

The backup MODE_INFO buffer used in the error concealment was
allocated in the codec common parts allocation even though this is a
decoder only resource. Moved the allocation to the decoder side.
No need to update_mode_info_border as mode_info buffers are zero
allocated.

This fixes also a potential memory leak as the EC overlaps buffer was not
properly released before reallocation after a frame size change.

Change-Id: I12803d3e012308d95669069980b1c95973fb775f

vp8/decoder/error_concealment.c

@@ -51,12 +51,13 @@ int vp8_alloc_overlap_lists(VP8D_COMP *pbi)
         vpx_free(pbi->overlaps);
         pbi->overlaps = NULL;
     }
+
     pbi->overlaps = vpx_calloc(pbi->common.mb_rows * pbi->common.mb_cols,
                                sizeof(MB_OVERLAP));
+
     if (pbi->overlaps == NULL)
         return -1;
-    vpx_memset(pbi->overlaps, 0,
-               sizeof(MB_OVERLAP) * pbi->common.mb_rows * pbi->common.mb_cols);
+
     return 0;
 }