Fix NPE in FakeDataSource.close()

If open() fails because of the file isn't available then fakeData is null.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=160249214

testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeDataSource.java

@@ -199,7 +199,7 @@ public final class FakeDataSource implements DataSource {
     Assertions.checkState(opened);
     opened = false;
     uri = null;
-    if (currentSegmentIndex < fakeData.segments.size()) {
+    if (fakeData != null && currentSegmentIndex < fakeData.segments.size()) {
       Segment current = fakeData.segments.get(currentSegmentIndex);
       if (current.isErrorSegment() && current.exceptionThrown) {
         current.exceptionCleared = true;