Fix NPE in TextTrackRenderer

Issue: #2081

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=139566990

library/src/main/java/com/google/android/exoplayer2/text/TextRenderer.java

@@ -114,19 +114,11 @@ public final class TextRenderer extends BaseRenderer implements Callback {
 
   @Override
   protected void onPositionReset(long positionUs, boolean joining) {
-    inputStreamEnded = false;
-    outputStreamEnded = false;
-    if (subtitle != null) {
-      subtitle.release();
-      subtitle = null;
-    }
-    if (nextSubtitle != null) {
-      nextSubtitle.release();
-      nextSubtitle = null;
-    }
-    nextInputBuffer = null;
     clearOutput();
+    resetBuffers();
     decoder.flush();
+    inputStreamEnded = false;
+    outputStreamEnded = false;
   }
 
   @Override
@@ -220,18 +212,10 @@ public final class TextRenderer extends BaseRenderer implements Callback {
 
   @Override
   protected void onDisabled() {
-    if (subtitle != null) {
-      subtitle.release();
-      subtitle = null;
-    }
-    if (nextSubtitle != null) {
-      nextSubtitle.release();
-      nextSubtitle = null;
-    }
+    clearOutput();
+    resetBuffers();
     decoder.release();
     decoder = null;
-    nextInputBuffer = null;
-    clearOutput();
     super.onDisabled();
   }
 
@@ -247,6 +231,19 @@ public final class TextRenderer extends BaseRenderer implements Callback {
     return true;
   }
 
+  private void resetBuffers() {
+    nextInputBuffer = null;
+    nextSubtitleEventIndex = C.INDEX_UNSET;
+    if (subtitle != null) {
+      subtitle.release();
+      subtitle = null;
+    }
+    if (nextSubtitle != null) {
+      nextSubtitle.release();
+      nextSubtitle = null;
+    }
+  }
+
   private long getNextEventTime() {
     return ((nextSubtitleEventIndex == C.INDEX_UNSET)
         || (nextSubtitleEventIndex >= subtitle.getEventTimeCount())) ? Long.MAX_VALUE