Fixed build on Mac, where g++ is a bit too dumb to see that a variable is always initialized.

src/ia32/code-stubs-ia32.cc

Index: src/ia32/code-stubs-ia32.cc
===================================================================
--- src/ia32/code-stubs-ia32.cc	(revision 7707)
+++ src/ia32/code-stubs-ia32.cc	(working copy)
@@ -533,7 +533,7 @@
       kMaxNameLength);
   if (name_ == NULL) return "OOM";
   const char* op_name = Token::Name(op_);
-  const char* overwrite_name;
+  const char* overwrite_name = NULL;  // Make g++ on Mac happy.
   switch (mode_) {
     case UNARY_NO_OVERWRITE: overwrite_name = "Alloc"; break;
     case UNARY_OVERWRITE: overwrite_name = "Overwrite"; break;