X64: Fix bug in boolean conversion of empty string.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index d813ed1dfd3e8695f0eaf3257f432d5074aa4f9b..7c9177859f1ae8688757d48714a6c5a52c9ec29f 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -1669,7 +1669,7 @@ Object* Heap::AllocateSlicedString(String* buffer,
   int length = end - start;
 
   // If the resulting string is small make a sub string.
-  if (end - start <= String::kMinNonFlatLength) {
+  if (length <= String::kMinNonFlatLength) {
     return Heap::AllocateSubString(buffer, start, end);
   }