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().

runtime/doc/eval.txt

@@ -8737,6 +8737,7 @@ test_override({name}, {val})				*test_override()*
 
 		name	     effect when {val} is non-zero ~
 		redraw       disable the redrawing() function
+		redraw_flag  ignore the RedrawingDisabled flag
 		char_avail   disable the char_avail() function
 		starting     reset the "starting" variable, see below
 		nfa_fail     makes the NFA regexp engine fail to force a