term: cursor_refresh(): don't try to dirty the grid if we don't have one

If the compositor sends a keyboard enter event before our window has
been mapped, foot crashes; the enter event triggers a cursor
refresh (hollow -> non-hollow block cursor), which crashes since we
haven't yet allocated a grid.

Fix by no-op:ing the refresh if the window hasn't been configured yet.

Closes #1910

CHANGELOG.md

@@ -74,6 +74,10 @@
 * 'flash' overlay (triggered by either `tput flash`, or enabling
   `bell.visual` and then sending `BEL` to the terminal) stuck when
   `colors.flash-alpha=1.0`.
+* Crash when compositor sends a keyboard enter event before the foot
+  window has been mapped ([#1910][1910]).
+
+[1910]: https://codeberg.org/dnkl/foot/issues/1910
 
 
 ### Security

terminal.c

@@ -515,6 +515,9 @@ term_arm_blink_timer(struct terminal *term)
 static void
 cursor_refresh(struct terminal *term)
 {
+    if (!term->window->is_configured)
+        return;
+
     term->grid->cur_row->cells[term->grid->cursor.point.col].attrs.clean = 0;
     term->grid->cur_row->dirty = true;
     render_refresh(term);