Fix build warnings

Change-Id: I334e0a6e018bb1067c90e050cf3583c1057aacd9
diff --git a/core/jni/android_util_AssetManager.cpp b/core/jni/android_util_AssetManager.cpp
index 6ecee35..17130af 100644
--- a/core/jni/android_util_AssetManager.cpp
+++ b/core/jni/android_util_AssetManager.cpp
@@ -434,12 +434,12 @@
 {
     ScopedUtfChars path8(env, path);
     if (path8.c_str() == NULL) {
-        return NULL;
+        return 0;
     }
 
     AssetManager* am = assetManagerForJavaObject(env, clazz);
     if (am == NULL) {
-        return JNI_FALSE;
+        return 0;
     }
 
     void* cookie;