Harden NumberToSize against overflows.

The callers to NumberToSize are supposed to validate the number, but
this adds a last line of defense.

R=jkummerow@chromium.org, ulan@chromium.org

Review URL: https://codereview.chromium.org/72323003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17733 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/v8conversions.h

@@ -60,10 +60,15 @@ inline size_t NumberToSize(Isolate* isolate,
                            Object* number) {
   SealHandleScope shs(isolate);
   if (number->IsSmi()) {
-    return Smi::cast(number)->value();
+    int value = Smi::cast(number)->value();
+    CHECK_GE(value, 0);
+    ASSERT(Smi::kMaxValue <= std::numeric_limits<size_t>::max());
+    return static_cast<size_t>(value);
   } else {
     ASSERT(number->IsHeapNumber());
     double value = HeapNumber::cast(number)->value();
+    CHECK(value >= 0 &&
+          value <= std::numeric_limits<size_t>::max());
     return static_cast<size_t>(value);
   }
 }