Avoid pointer underflow in CopyCharsUnsigned.

src/v8utils.h

Index: src/v8utils.h
diff --git a/src/v8utils.h b/src/v8utils.h
index 2064c5ac2c97d9d7759009bc79aade7493a3f746..3a9b776849652f23bca2f74545c901dd0819ac3a 100644
--- a/src/v8utils.h
+++ b/src/v8utils.h
@@ -249,7 +249,8 @@ void CopyCharsUnsigned(sinkchar* dest, const sourcechar* src, int chars) {
     }
     // Number of characters in a uintptr_t.
     static const int kStepSize = sizeof(uintptr_t) / sizeof(*dest);  // NOLINT
-    while (dest <= limit - kStepSize) {
+    ASSERT(dest + kStepSize > dest);  // Check for overflow.
+    while (dest + kStepSize <= limit) {
       *reinterpret_cast<uintptr_t*>(dest) =
           *reinterpret_cast<const uintptr_t*>(src);
       dest += kStepSize;