Changes to compile with gcc 4.6

Change-Id: I55908b3b98b49373ce948fd1f12e0a5cd56ae4f7
diff --git a/src/dalvik_system_DexFile.cc b/src/dalvik_system_DexFile.cc
index 23247cf..7fce25f 100644
--- a/src/dalvik_system_DexFile.cc
+++ b/src/dalvik_system_DexFile.cc
@@ -97,7 +97,7 @@
     if (!IsValidZipFilename(sourceName.c_str()) || !IsValidDexFilename(outputName.c_str())) {
       LOG(ERROR) << "Bad filenames extracting dex '" << outputName.c_str()
                  << "' from zip '" << sourceName.c_str() << "'";
-      return NULL;
+      return 0;
     }
     // Generate the output oat file for the source dex file
     ClassLinker* class_linker = Runtime::Current()->GetClassLinker();