vim-patch:8.1.1579: dict and list could be GC'ed while displaying error

Problem:    Dict and list could be GC'ed while displaying error in a timer.
            (Yasuhiro Matsumoto)
Solution:   Block garbage collection when executing a timer.  Add
            test_garbagecollect_soon(). Add "no_wait_return" to
            test_override(). (closes vim/vim#4571)
adc6714aac

src/nvim/getchar.c

@@ -1518,7 +1518,7 @@ int vgetc(void)
    * collection in the first next vgetc().  It's disabled after that to
    * avoid internally used Lists and Dicts to be freed.
    */
-  may_garbage_collect = FALSE;
+  may_garbage_collect = false;
 
   return c;
 }