Check thread is still alive before sending message in Loader.

The release callback handler in Loader might not be alive
anymore. Catch this case to prevent warnings about sending
messages on dead threads.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=167996538

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

@@ -420,7 +420,9 @@ public final class Loader implements LoaderErrorThrower {
 
     @Override
     public void run() {
-      sendEmptyMessage(0);
+      if (getLooper().getThread().isAlive()) {
+        sendEmptyMessage(0);
+      }
     }
 
     @Override