vim-patch:8.1.0868: crash if triggering garbage collector after a function call

Problem:    Crash if triggering garbage collector after a function call.
            (Michael Henry)
Solution:   Don't call the garbage collector right away, do it later.
            (closes vim/vim#3894)
889da2f243

src/nvim/eval/userfunc.c

@@ -613,10 +613,10 @@ static void cleanup_function_call(funccall_T *fc)
     if (++made_copy == 10000) {
       // We have made a lot of copies.  This can happen when
       // repetitively calling a function that creates a reference to
-      // itself somehow.  Call the garbage collector here to avoid using
+      // itself somehow.  Call the garbage collector soon to avoid using
       // too much memory.
       made_copy = 0;
-      (void)garbage_collect(false);
+      want_garbage_collect = true;
     }
   }
 }