Work around a bug in Clang that optimizes away a NULL check

src/spaces.h

Index: src/spaces.h
diff --git a/src/spaces.h b/src/spaces.h
index 25f907ebd8a1653247d564bf49a2aa0fb29420ad..9cab8beef26ae9411ca5da87f7c6d320fd7c738f 100644
--- a/src/spaces.h
+++ b/src/spaces.h
@@ -333,6 +333,14 @@ class MemoryChunk {
            kFailureTag);
   }
 
+  // Workaround for a bug in Clang-3.3 which in some situations optimizes away
+  // an "if (chunk->owner() != NULL)" check.
+  bool has_owner() {
+   if (owner_ == 0) return false;
+   if (reinterpret_cast<intptr_t>(owner_) == kFailureTag) return false;
+   return true;
+  }
+
   VirtualMemory* reserved_memory() {
     return &reservation_;
   }