Fix VP8 reader fragment packet size

Merging https://github.com/androidx/media/pull/110/commits/c37a767fdfc2f7e0b9eb1ad7ae6f864c93950712

PiperOrigin-RevId: 463826373
(cherry picked from commit 4d5d592c)

library/rtsp/src/main/java/com/google/android/exoplayer2/source/rtsp/reader/RtpVp8Reader.java

@@ -214,7 +214,7 @@ import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
             fragmentedSampleSizeBytes,
             /* offset= */ 0,
             /* cryptoData= */ null);
-    fragmentedSampleSizeBytes = 0;
+    fragmentedSampleSizeBytes = C.LENGTH_UNSET;
     fragmentedSampleTimeUs = C.TIME_UNSET;
     gotFirstPacketOfVp8Frame = false;
   }