Fix compiler warnings in frameworks/av/media/ndk.

Bug: 26936282
Change-Id: I86b32ef5a7e6b18974cbe4a885291acec2b2eb5f
diff --git a/media/ndk/NdkImageReader.cpp b/media/ndk/NdkImageReader.cpp
index d57a86e..387a302 100644
--- a/media/ndk/NdkImageReader.cpp
+++ b/media/ndk/NdkImageReader.cpp
@@ -203,10 +203,14 @@
 
     mCbLooper = new ALooper;
     mCbLooper->setName(consumerName.string());
-    status_t ret = mCbLooper->start(
+    res = mCbLooper->start(
             /*runOnCallingThread*/false,
             /*canCallJava*/       true,
             PRIORITY_DEFAULT);
+    if (res != OK) {
+        ALOGE("Failed to start the looper");
+        return AMEDIA_ERROR_UNKNOWN;
+    }
     mHandler = new CallbackHandler(this);
     mCbLooper->registerHandler(mHandler);