patch 8.1.0349: crash when wiping buffer in a callback

Problem:    Crash when wiping buffer in a callback.
Solution:   Do not handle messages when only peeking for a character.
            (closes #2107)  Add "redraw_flag" to test_override().

src/screen.c

@@ -10819,8 +10819,11 @@ redrawing(void)
 	return 0;
     else
 #endif
-	return (!RedrawingDisabled
-		       && !(p_lz && char_avail() && !KeyTyped && !do_redraw));
+	return ((!RedrawingDisabled
+#ifdef FEAT_EVAL
+		    || ignore_redraw_flag_for_testing
+#endif
+		) && !(p_lz && char_avail() && !KeyTyped && !do_redraw));
 }
 
 /*