Add unit test for FakeDataSource.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=163341994

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

@@ -162,8 +162,8 @@ public class FakeDataSet {
     }
 
     /**
-     * Appends data of the specified length. No actual data is available and this data should not
-     * be read.
+     * Appends a data segment of the specified length. No actual data is available and the
+     * {@link FakeDataSource} will perform no copy operations when this data is read.
      */
     public FakeData appendReadData(int length) {
       Assertions.checkState(length > 0);

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

@@ -166,6 +166,7 @@ public class FakeDataSource implements DataSource {
         // Do not allow crossing of the segment boundary.
         readLength = Math.min(readLength, current.length - current.bytesRead);
         // Perform the read and return.
+        Assertions.checkArgument(buffer.length - offset >= readLength);
         if (current.data != null) {
           System.arraycopy(current.data, current.bytesRead, buffer, offset, readLength);
         }