Added clamp of qindex to valid range

The qindex for a segment was not clamped in ABSDATA mode, which may
cause invalid memory access if an ill-formed stream has a negative
value in ABSDATA mode. This commit added clamp to make sure qindex
for a segment always fall into valid range.

Change-Id: I0a74d00f4ef40aec7edaeca1d03c8645e23ab08c

vp9/common/vp9_quant_common.c

@@ -134,9 +134,9 @@ int vp9_get_qindex(const struct segmentation *seg, int segment_id,
                    int base_qindex) {
   if (vp9_segfeature_active(seg, segment_id, SEG_LVL_ALT_Q)) {
     const int data = vp9_get_segdata(seg, segment_id, SEG_LVL_ALT_Q);
-    return seg->abs_delta == SEGMENT_ABSDATA ?
-                             data :  // Abs value
-                             clamp(base_qindex + data, 0, MAXQ);  // Delta value
+    const int seg_qindex = seg->abs_delta == SEGMENT_ABSDATA ?
+        data : base_qindex + data;
+    return clamp(seg_qindex, 0, MAXQ);
   } else {
     return base_qindex;
   }