Don't crash during full backup when CheckJNI is enabled

It's not okay to pass null to JNI methods and trust that it will just
back off and return a null result cleanly.  Fixes bug 5361822 .

Change-Id: Id8a17b958fd183d55cb6475f394e158c13aae2ea

Author: Christopher Tate

core/jni/android_app_backup_FullBackup.cpp

@@ -76,10 +76,10 @@ static int backupToTar(JNIEnv* env, jobject clazz, jstring packageNameObj,
     int ret;
 
     // Extract the various strings, allowing for null object pointers
-    const char* packagenamechars = env->GetStringUTFChars(packageNameObj, NULL);
-    const char* rootchars = env->GetStringUTFChars(rootpathObj, NULL);
-    const char* pathchars = env->GetStringUTFChars(pathObj, NULL);
-    const char* domainchars = env->GetStringUTFChars(domainObj, NULL);
+    const char* packagenamechars = (packageNameObj) ? env->GetStringUTFChars(packageNameObj, NULL) : NULL;
+    const char* rootchars = (rootpathObj) ? env->GetStringUTFChars(rootpathObj, NULL) : NULL;
+    const char* pathchars = (pathObj) ? env->GetStringUTFChars(pathObj, NULL) : NULL;
+    const char* domainchars = (domainObj) ? env->GetStringUTFChars(domainObj, NULL) : NULL;
 
     String8 packageName(packagenamechars ? packagenamechars : "");
     String8 rootpath(rootchars ? rootchars : "");