Fix bug when calculating EOF position in mp4 sniffing

PiperOrigin-RevId: 227668426

library/core/src/main/java/com/google/android/exoplayer2/extractor/mp4/Sniffer.java

@@ -114,7 +114,7 @@ import java.io.IOException;
         // The atom extends to the end of the file.
         long endPosition = input.getLength();
         if (endPosition != C.LENGTH_UNSET) {
-          atomSize = endPosition - input.getPosition() + headerSize;
+          atomSize = endPosition - input.getPeekPosition() + headerSize;
         }
       }