Fix seeking to a non-loaded period.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=127536151

library/src/main/java/com/google/android/exoplayer2/ExoPlayerImplInternal.java

@@ -809,10 +809,11 @@ import java.util.ArrayList;
         maybeContinueLoading();
       } else {
         for (Renderer renderer : enabledRenderers) {
-          ensureStopped(renderer);
           renderer.disable();
         }
         enabledRenderers = new Renderer[0];
+        rendererMediaClock = null;
+        rendererMediaClockSource = null;
         playingPeriod = null;
         readingPeriod = null;
         loadingPeriod = null;