Fix build warnings

Change-Id: I33178dbe0bfc087d6599579ca0529ad853c669ed
diff --git a/media/libnbaio/PipeReader.cpp b/media/libnbaio/PipeReader.cpp
index b096903..a879647 100644
--- a/media/libnbaio/PipeReader.cpp
+++ b/media/libnbaio/PipeReader.cpp
@@ -36,7 +36,12 @@
 
 PipeReader::~PipeReader()
 {
-    int32_t readers = android_atomic_dec(&mPipe.mReaders);
+#if !LOG_NDEBUG
+    int32_t readers =
+#else
+    (void)
+#endif
+            android_atomic_dec(&mPipe.mReaders);
     ALOG_ASSERT(readers > 0);
 }