Fix potential segfault on ENOENT on job file

Introduced in commit 323255cc2269, if we fail opening, td may
not be set.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/init.c b/init.c
index a2377dc..1768478 100644
--- a/init.c
+++ b/init.c
@@ -1432,7 +1432,8 @@
 			int __err = errno;
 
 			log_err("fio: unable to open '%s' job file\n", file);
-			td_verror(td, __err, "job file open");
+			if (td)
+				td_verror(td, __err, "job file open");
 			return 1;
 		}
 	}