Fix compile error on gcc-4.5 bug 963

Review URL: http://codereview.chromium.org/5642004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5942 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/spaces.cc

@@ -917,7 +917,7 @@ AtomicWord* MemoryAllocator::AllocatedChunksFinder(
     }
     more_fine_grained_table = new AtomicWord[words_needed];
     for (int i = 0; i < words_needed; i++) {
-      more_fine_grained_table[i] = NULL;
+      more_fine_grained_table[i] = kUnusedChunkTableEntry;
     }
     Release_Store(&table[index],
                   reinterpret_cast<AtomicWord>(more_fine_grained_table));