Avoid signaling errors from 'pixel-fill-region'

* lisp/textmodes/pixel-fill.el (pixel-fill-region): Make sure the
selected window displays the current buffer.  This is important
when this function is called inside 'with-current-buffer' or
similar forms which temporarily change the buffer displayed in the
selected window.  (Bug#67791)

lisp/textmodes/pixel-fill.el

@@ -73,6 +73,8 @@ lines that are visually wider than PIXEL-WIDTH.
 If START isn't at the start of a line, the horizontal position of
 START, converted to pixel units, will be used as the indentation
 prefix on subsequent lines."
+  (save-window-excursion
+    (set-window-buffer nil (current-buffer))
     (save-excursion
       (goto-char start)
       (let ((indentation
@@ -105,7 +107,7 @@ prefix on subsequent lines."
           (pixel-fill--fill-line pixel-width indentation)
           (goto-char (point-max))
           (when newline-end
-          (insert "\n"))))))
+            (insert "\n")))))))
 
 (defun pixel-fill--goto-pixel (width)
   (vertical-motion (cons (/ width (frame-char-width)) 0)))