Prevent access of array using -1 index

BUG=aomedia:685

Change-Id: I0698b281fbaff2e77a50f5dc9a10d752de6c6f79

av1/decoder/decodeframe.c

@@ -2139,7 +2139,7 @@ static PARTITION_TYPE read_partition(AV1_COMMON *cm, MACROBLOCKD *xd,
       partition_plane_context(xd, mi_row, mi_col, has_rows, has_cols, bsize);
   const aom_prob *const probs =
       ctx < PARTITION_CONTEXTS ? cm->fc->partition_prob[ctx] : NULL;
-  FRAME_COUNTS *const counts = ctx < PARTITION_CONTEXTS ? xd->counts : NULL;
+  FRAME_COUNTS *const counts = ctx >= 0 ? xd->counts : NULL;
 #else
   const int ctx = partition_plane_context(xd, mi_row, mi_col, bsize);
   const aom_prob *const probs = cm->fc->partition_prob[ctx];

av1/encoder/encodeframe.c

@@ -3304,7 +3304,7 @@ static void rd_pick_partition(const AV1_COMP *const cpi, ThreadData *td,
 #endif
                                          bsize);
 #endif  // CONFIG_CB4X4
-  const int *partition_cost = x->partition_cost[pl];
+  const int *partition_cost = pl >= 0 ? x->partition_cost[pl] : NULL;
 #if CONFIG_SUPERTX
   int this_rate_nocoef, sum_rate_nocoef = 0, best_rate_nocoef = INT_MAX;
   int abort_flag;