Fix missing release calls on loadingPeriodHolder

playingPeriodHolder can be null in the case that the first
period is still being prepared. We need to make sure we
release the period that's being prepared in such cases,
which is loadingPeriodHolder.

Issue: #1914

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=135793472

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

@@ -538,16 +538,23 @@ private long seekToPeriodPosition(int periodIndex, long periodPositionUs)
       periodIndex = C.INDEX_UNSET;
     }
 
-    // Clear the timeline, but keep the requested period if it is already prepared.
-    MediaPeriodHolder<T> periodHolder = playingPeriodHolder;
     MediaPeriodHolder<T> newPlayingPeriodHolder = null;
-    while (periodHolder != null) {
-      if (periodHolder.index == periodIndex && periodHolder.prepared) {
-        newPlayingPeriodHolder = periodHolder;
-      } else {
-        periodHolder.release();
+    if (playingPeriodHolder == null) {
+      // We're still waiting for the first period to be prepared.
+      if (loadingPeriodHolder != null) {
+        loadingPeriodHolder.release();
+      }
+    } else {
+      // Clear the timeline, but keep the requested period if it is already prepared.
+      MediaPeriodHolder<T> periodHolder = playingPeriodHolder;
+      while (periodHolder != null) {
+        if (periodHolder.index == periodIndex && periodHolder.prepared) {
+          newPlayingPeriodHolder = periodHolder;
+        } else {
+          periodHolder.release();
+        }
+        periodHolder = periodHolder.next;
       }
-      periodHolder = periodHolder.next;
     }
 
     // Disable all the renderers if the period is changing.
@@ -892,7 +899,8 @@ private void attemptRestart(Timeline newTimeline, Timeline oldTimeline,
     }
 
     // Release all loaded periods.
-    releasePeriodHoldersFrom(playingPeriodHolder);
+    releasePeriodHoldersFrom(playingPeriodHolder != null ? playingPeriodHolder
+        : loadingPeriodHolder);
     bufferAheadPeriodCount = 0;
     playingPeriodHolder = null;
     readingPeriodHolder = null;