Fix gcc 4.6 build

Change-Id: I254802b426cb8636330fad6954df251c00cb9ee5
diff --git a/src/compiled_method.cc b/src/compiled_method.cc
index 4e21a91..67bdd9a 100644
--- a/src/compiled_method.cc
+++ b/src/compiled_method.cc
@@ -110,7 +110,7 @@
     }
     default:
       LOG(FATAL) << "Unknown InstructionSet " << (int) instruction_set_;
-      return NULL;
+      return 0;
   }
 }