Fix possible media extractor service crash

        case MAKE_EXTRACTOR: {

            CHECK_INTERFACE(IMediaExtractorService, data, reply);

            sp<IDataSource> source = interface_cast<IDataSource>(data.readStrongBinder());

            sp<IBinder> b;

            status_t ret = data.readStrongBinder(&b);

            if (ret != NO_ERROR || b == NULL) {

                ALOGE("Error reading source from parcel");

                return ret;

            }

            sp<IDataSource> source = interface_cast<IDataSource>(b);

            const char *mime = data.readCString();

            sp<IMediaExtractor> ex = makeExtractor(source, mime);

            reply->writeStrongBinder(IInterface::asBinder(ex));