Bufferpool: Check hidl call return value.

Hidl will crash if hidl call failed and return value was not checked.
Check hidl call return values.

Bug: 132182836
Change-Id: I910025a7b3c508b3c8f289a5286abbcc77e2d9b8
diff --git a/media/bufferpool/2.0/AccessorImpl.cpp b/media/bufferpool/2.0/AccessorImpl.cpp
index 5260909..94cf006 100644
--- a/media/bufferpool/2.0/AccessorImpl.cpp
+++ b/media/bufferpool/2.0/AccessorImpl.cpp
@@ -261,13 +261,19 @@
         mBufferPool.mInvalidation.onHandleAck(&observers, &invalidationId);
     }
     // Do not hold lock for send invalidations
+    size_t deadClients = 0;
     for (auto it = observers.begin(); it != observers.end(); ++it) {
         const sp<IObserver> observer = it->second;
         if (observer) {
             Return<void> transResult = observer->onMessage(it->first, invalidationId);
-            (void) transResult;
+            if (!transResult.isOk()) {
+                ++deadClients;
+            }
         }
     }
+    if (deadClients > 0) {
+        ALOGD("During invalidation found %zu dead clients", deadClients);
+    }
 }
 
 bool Accessor::Impl::isValid() {