render: xcursor_is_valid(): don't crash when there's no theme loaded

When trying to set a custom cursor shape, we first validate it. This
is done by checking against known server-side names, and then trying
to load the cursor from the client side cursor theme.

But, if we're using server side names, there is no theme loaded, and
foot crashed.

Fix by checking if we have a theme loaded, and if not, fail the cursor
shape name validation.

CHANGELOG.md

@@ -98,6 +98,8 @@
 * "Too large" values for `scrollback.lines` causing an integer
   overflow, resulting in either visual glitches, crashes, or both
   ([#1828][1828]).
+* Crash when trying to set an invalid cursor shape with OSC-22, when
+  foot uses server-side cursor shapes.
 
 [1828]: https://codeberg.org/dnkl/foot/issues/1828
 

render.c

@@ -4722,6 +4722,8 @@ render_xcursor_is_valid(const struct seat *seat, const char *cursor)
 {
     if (cursor == NULL)
         return false;
+    if (seat->pointer.theme == NULL)
+        return false;
     return wl_cursor_theme_get_cursor(seat->pointer.theme, cursor) != NULL;
 }