Fix the case of no words to copy.

src/utils.h

Index: src/utils.h
diff --git a/src/utils.h b/src/utils.h
index ba27f07290b8f23803581e82ab295629fc03f1fc..b123057322403f53341a5f982de315d8e1756858 100644
--- a/src/utils.h
+++ b/src/utils.h
@@ -600,6 +600,8 @@ static inline void MemsetPointer(T** dest, T* value, int counter) {
 // Copies data from |src| to |dst|.  The data spans MUST not overlap.
 inline void CopyWords(Object** dst, Object** src, int num_words) {
   ASSERT(Min(dst, src) + num_words <= Max(dst, src));
+  ASSERT(num_words > 0);
+
   // Use block copying memcpy if the segment we're copying is
   // enough to justify the extra call/setup overhead.
   static const int kBlockCopyLimit = 16;