Fix possible track selection NPE

If no tracks are selected at the start of playback, TrackSelectorResult
isEquivalent(null) returned true, meaning we were keeping the old result
(i.e. null), which we then tried to de-reference.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=145105702

library/src/main/java/com/google/android/exoplayer2/trackselection/TrackSelectorResult.java

@@ -61,10 +61,14 @@ public final class TrackSelectorResult {
   /**
   /**
    * Returns whether this result is equivalent to {@code other} for all renderers.
    * Returns whether this result is equivalent to {@code other} for all renderers.
    *
    *
-   * @param other The other {@link TrackSelectorResult}. May be null.
+   * @param other The other {@link TrackSelectorResult}. May be null, in which case {@code false}
+   *     will be returned in all cases.
    * @return Whether this result is equivalent to {@code other} for all renderers.
    * @return Whether this result is equivalent to {@code other} for all renderers.
    */
    */
   public boolean isEquivalent(TrackSelectorResult other) {
   public boolean isEquivalent(TrackSelectorResult other) {
+    if (other == null) {
+      return false;
+    }
     for (int i = 0; i < selections.length; i++) {
     for (int i = 0; i < selections.length; i++) {
       if (!isEquivalent(other, i)) {
       if (!isEquivalent(other, i)) {
         return false;
         return false;
@@ -78,13 +82,14 @@ public final class TrackSelectorResult {
    * The results are equivalent if they have equal track selections and configurations for the
    * The results are equivalent if they have equal track selections and configurations for the
    * renderer.
    * renderer.
    *
    *
-   * @param other The other {@link TrackSelectorResult}. May be null.
+   * @param other The other {@link TrackSelectorResult}. May be null, in which case {@code false}
+   *     will be returned in all cases.
    * @param index The renderer index to check for equivalence.
    * @param index The renderer index to check for equivalence.
    * @return Whether this result is equivalent to {@code other} for all renderers.
    * @return Whether this result is equivalent to {@code other} for all renderers.
    */
    */
   public boolean isEquivalent(TrackSelectorResult other, int index) {
   public boolean isEquivalent(TrackSelectorResult other, int index) {
     if (other == null) {
     if (other == null) {
-      return selections.get(index) == null && rendererConfigurations[index] == null;
+      return false;
     }
     }
     return Util.areEqual(selections.get(index), other.selections.get(index))
     return Util.areEqual(selections.get(index), other.selections.get(index))
         && Util.areEqual(rendererConfigurations[index], other.rendererConfigurations[index]);
         && Util.areEqual(rendererConfigurations[index], other.rendererConfigurations[index]);