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/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 6408165..d51b92c 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -8737,6 +8737,7 @@
 
 		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