Reenable ability to Dispose already Disposed Vector.

Review URL: http://codereview.chromium.org/1016006

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4170 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/utils.h

@@ -342,7 +342,6 @@ class Vector {
   // vector is empty.
   void Dispose() {
     DeleteArray(start_);
-    if (is_empty()) return;
     start_ = NULL;
     length_ = 0;
   }