Be more robust against load callback failures

Issue: #2795

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=174836960

library/core/src/main/java/com/google/android/exoplayer2/upstream/Loader.java

@@ -380,7 +380,13 @@ public void handleMessage(Message msg) {
           callback.onLoadCanceled(loadable, nowMs, durationMs, false);
           break;
         case MSG_END_OF_SOURCE:
-          callback.onLoadCompleted(loadable, nowMs, durationMs);
+          try {
+            callback.onLoadCompleted(loadable, nowMs, durationMs);
+          } catch (RuntimeException e) {
+            // This should never happen, but handle it anyway.
+            Log.e(TAG, "Unexpected exception handling load completed", e);
+            fatalError = new UnexpectedLoaderException(e);
+          }
           break;
         case MSG_IO_EXCEPTION:
           currentError = (IOException) msg.obj;
@@ -392,6 +398,9 @@ public void handleMessage(Message msg) {
             start(getRetryDelayMillis());
           }
           break;
+        default:
+          // Never happens.
+          break;
       }
     }