Fix continueLoading in IMA deferred periods

continueLoading may be called during preparation, but this is not
handled correctly in the case where a deferred period doesn't
have a source yet.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=158696539

extensions/ima/src/main/java/com/google/android/exoplayer2/ext/ima/ImaAdsMediaSource.java

@@ -506,7 +506,7 @@ public final class ImaAdsMediaSource implements MediaSource {
 
     @Override
     public boolean continueLoading(long positionUs) {
-      return mediaPeriod.continueLoading(positionUs);
+      return mediaPeriod != null && mediaPeriod.continueLoading(positionUs);
     }
 
     // MediaPeriod.Callback implementation.