jffs2: Fix rtime decompressor

The fix for a memory corruption contained a off-by-one error and
caused the compressor to fail in legit cases.

Cc: Kinsey Moore <kinsey.moore@oarcorp.com>
Cc: stable@vger.kernel.org
Fixes: fe051552 ("jffs2: Prevent rtime decompress memory corruption")
Signed-off-by: Richard Weinberger <richard@nod.at>