Increase buffer size for saving menucolors

The '"regex"=color&attr' is max BUFSZ, so prefixing with MENUCOLOR=
and adding a newline overflowed the temp buffer when saving the
menucolors.

src/options.c

@@ -8788,7 +8788,7 @@ all_options_menucolors(strbuf_t *sbuf)
 {
     int i = 0, ncolors = count_menucolors();
     struct menucoloring *tmp = g.menu_colorings;
-    char buf[BUFSZ];
+    char buf[BUFSZ*2]; /* see also: add_menu_coloring() */
     struct menucoloring **arr;
 
     if (!ncolors)