Don't discard when a reset is pending

When a reset is pending the sample queues normally contain pre-seek
samples (which will be emptied when a pending load is canceled). The
position passed to discard will be a post-seek position. I don't
think anything bad happened in any of the cases being changed, but
discarding is unnecessary in such cases, and reasoning about such
cases is difficult.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=212435556

library/core/src/main/java/com/google/android/exoplayer2/source/ExtractorMediaPeriod.java

@@ -291,6 +291,9 @@ import org.checkerframework.checker.nullness.compatqual.NullableType;
 
   @Override
   public void discardBuffer(long positionUs, boolean toKeyframe) {
+    if (isPendingReset()) {
+      return;
+    }
     boolean[] trackEnabledStates = getPreparedState().trackEnabledStates;
     int trackCount = sampleQueues.length;
     for (int i = 0; i < trackCount; i++) {

library/core/src/main/java/com/google/android/exoplayer2/source/chunk/ChunkSampleStream.java

@@ -188,6 +188,9 @@ public class ChunkSampleStream<T extends ChunkSource> implements SampleStream, S
    *     the specified position, rather than any sample before or at that position.
    */
   public void discardBuffer(long positionUs, boolean toKeyframe) {
+    if (isPendingReset()) {
+      return;
+    }
     int oldFirstIndex = primarySampleQueue.getFirstIndex();
     primarySampleQueue.discardTo(positionUs, toKeyframe, true);
     int newFirstIndex = primarySampleQueue.getFirstIndex();

library/hls/src/main/java/com/google/android/exoplayer2/source/hls/HlsSampleStreamWrapper.java

@@ -367,7 +367,7 @@ import java.util.List;
   }
 
   public void discardBuffer(long positionUs, boolean toKeyframe) {
-    if (!sampleQueuesBuilt) {
+    if (!sampleQueuesBuilt || isPendingReset()) {
       return;
     }
     int sampleQueueCount = sampleQueues.length;