Properly initialize shadow_fd to -1

We forget that in one case, causing all sorts of issues when
we end up closing this bogus (STDIN) descriptor.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/filesetup.c b/filesetup.c
index db80a8b..fd55cc4 100644
--- a/filesetup.c
+++ b/filesetup.c
@@ -1421,6 +1421,7 @@
 			assert(0);
 		}
 		__f->fd = -1;
+		__f->shadow_fd = -1;
 		fio_file_reset(td, __f);
 
 		if (f->file_name) {