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().
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 8a1fcef..90fb888 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -13073,6 +13073,8 @@
 
 	if (STRCMP(name, (char_u *)"redraw") == 0)
 	    disable_redraw_for_testing = val;
+	else if (STRCMP(name, (char_u *)"redraw_flag") == 0)
+	    ignore_redraw_flag_for_testing = val;
 	else if (STRCMP(name, (char_u *)"char_avail") == 0)
 	    disable_char_avail_for_testing = val;
 	else if (STRCMP(name, (char_u *)"starting") == 0)
@@ -13095,6 +13097,7 @@
 	{
 	    disable_char_avail_for_testing = FALSE;
 	    disable_redraw_for_testing = FALSE;
+	    ignore_redraw_flag_for_testing = FALSE;
 	    nfa_fail_for_testing = FALSE;
 	    if (save_starting >= 0)
 	    {