Workaround for gcc array bound check issue

V8 doesn't build on Ubuntu 16.04 (with GCC 5.3). Seems to be
a known regression on newer GCC version. It emits incorrect
"error: array subscript is above array bounds" message. Adding
explicit array bound check fixes the issue.

R=hablich@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2256113002
Cr-Commit-Position: refs/heads/master@{#38721}

src/heap/slot-set.h

@@ -194,9 +194,15 @@ class SlotSet : public Malloced {
   }
 
   void MaskCell(int bucket_index, int cell_index, uint32_t mask) {
-    uint32_t* cells = bucket[bucket_index];
-    if (cells != nullptr && cells[cell_index] != 0) {
-      cells[cell_index] &= mask;
+    if (bucket_index < kBuckets) {
+      uint32_t* cells = bucket[bucket_index];
+      if (cells != nullptr && cells[cell_index] != 0) {
+        cells[cell_index] &= mask;
+      }
+    } else {
+      // GCC bug 59124: Emits wrong warnings
+      // "array subscript is above array bounds"
+      UNREACHABLE();
     }
   }