Allow for mark == bytes.length. (elastic#128340) (elastic#128466)

In the case that the mark is at the very end of the stream,
the slice's stream could already be closed by the time we
call reset(), and so the stream must be reopened.

Author: prdoyle

server/src/test/java/org/elasticsearch/index/snapshots/blobstore/SlicedInputStreamTests.java

@@ -178,7 +178,8 @@ protected InputStream openSlice(int slice) throws IOException {
         // Reset
         input.reset();
         int slicesOpenedAfterReset = streamsOpened.size();
-        assert moreBytes > 0 || mark == 0 || slicesOpenedAfterReset == slicesOpenedAtMark : "Reset at mark should not re-open slices";
+        assert moreBytes > 0 || mark == 0 || mark == bytes.length || slicesOpenedAfterReset == slicesOpenedAtMark
+            : "Reset at mark should not re-open slices";
 
         // Read all remaining bytes, which should be the bytes from mark up to the end
         final int remainingBytes = bytes.length - mark;