Fix set fullscreen not working after upstream Vim merge

The recent upstream Vim merge (v9.0.1365) contained a lot of refactoring
in the option setting code. Handling option setting was now done via a
standardized callback, with a struct `optset_T`. In resolving the
upstream merge conflicts, `did_set_fullscreen` was erroneously using the
wrong value from the union (should be using boolean instead of number)
which leads to it occasionally giving unpredictable results. Fix the
bug.

Note that the bug seems a little unpredictable since it depends on
compiler specifics. The only way it was known to trigger was by `set
diff` followed by `set fullscreen`.

Fix #1378

src/option.c

@@ -3505,7 +3505,7 @@ did_set_textmode(optset_T *args)
     char *
 did_set_fullscreen(optset_T *args)
 {
-    long old_value = args->os_oldval.number;
+    int old_value = args->os_oldval.boolean;
     if (p_fullscreen && !old_value)
     {
 	guicolor_T fg, bg;