Immediately release temp buffer memory in FakeRenderer.

FakeRenderer only needs to allocate real memory because it extends BaseRenderer
which uses the actual SampleStream implementation. Immediately release the
memory after using it to prevent excessive memory usage when running fast
simulations.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177949628

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

@@ -59,10 +59,10 @@ public class FakeRenderer extends BaseRenderer {
   @Override
   public void render(long positionUs, long elapsedRealtimeUs) throws ExoPlaybackException {
     if (!isEnded) {
-      buffer.clear();
       // Verify the format matches the expected format.
       FormatHolder formatHolder = new FormatHolder();
       int result = readSource(formatHolder, buffer, false);
+      buffer.data = null;
       if (result == C.RESULT_FORMAT_READ) {
         formatReadCount++;
         Assert.assertTrue(expectedFormats.contains(formatHolder.format));