Fix effect-free warning in ob-screen.el while loop

The warning was introduced and detected by an optimizer addition
proposed in the following thread:
https://lists.gnu.org/archive/html/emacs-devel/2019-12/msg00711.html

* lisp/org/ob-screen.el (org-babel-screen-test): Avoid 'value
returned from (format "...") is unused' warning by doing something
more useful than busy string manipulation while waiting for an
asynchronous subprocess to make the temporary file readable.

1 files changed, 1 insertions, 1 deletions

diff --git a/lisp/org/ob-screen.el b/lisp/org/ob-screen.el
index ad00ee070d4..837c18f8407 100644
--- a/lisp/org/ob-screen.el
+++ b/lisp/org/ob-screen.el
@@ -126,7 +126,7 @@ The terminal should shortly flicker."
     ;; XXX: need to find a better way to do the following
     (while (not (file-readable-p tmpfile))
       ;; do something, otherwise this will be optimized away
-      (format "org-babel-screen: File not readable yet."))
+      (sit-for 0.1))
     (setq tmp-string (with-temp-buffer
                        (insert-file-contents-literally tmpfile)
                        (buffer-substring (point-min) (point-max))))