[Bug-readline] nul's from tputs(called by rl_ding) when prefer_visible_bell is On

[McConnaughey, Bill]

I made some additions to readline-6.1 in order to record what goes in and out, 
so that I can
debug a misbehaving graphical interface. The record reveals a probable bug, but 
I don't know
enough about curses to tell whether the bug is in there or in readline.

Consistently seen pattern, when prefer-visible-bell is On
Left column is from the log file made by readline with my changes; explanations 
are added at the right

i1 ^[                                   I pushed the cursor-left key
i1 [
i1 D
o3 tputs[1](^H)                         readline emitted a backspace
o1 ^H
i1 ^[                                   I pushed the cursor-left key, but I was 
already at the start of the line
i1 [
i1 D
o10 tputs(^[[?5h^[[?5l$<200/>)          readline issued the visible bell 
sequence via tputs in rl_ding
o1 ^[
o1 [
o1 ?
o1 5
o1 h
o1 ^[
o1 [
o1 ?
o1 5
o1 l
o1 ^@                                   and it is followed by exactly 853 nuls's
o1 ^@
o1 ^@
o1 ^@
.....
o1 ^@


In case you are interested in my readline logging modifications, here's some 
typical output
with comments added. It identifies the location in the source code by "i1", 
"o2", etc. and
shows the data being transferred. I'll send you the modified sources if you 
want.

i1 I                      I typed 'I' and it was received by rl_read_key
o2 I                      readline echo'd it via _rl_output_some_chars
i1                        ... and so on ...
o2
i1 t
o2 t
i1 s
o2 s
i1 ^M                     I hit the <return> key
o6                        readline sent a ^M via putc

r12<I typed this>         readline returned a 12-character string
o2


o2 ^[[7mexperix>^[[0m     readline issued the command prompt via 
_rl_output_some_chars
i1 ^R                     I typed ^R
o18 tputs(^M)             readline emitted ^M from _rl_move_cursor_relative
o1 ^M
o25 tputs(^[[13@)         readline emitted the 'ICH' control sequence from 
insert_some_chars
o1 ^[
o1 [
o1 1
o1 3
o1 @
o2 (reverse-i-se          readline emitted this prompt from 
_rl_output_some_chars
o2 arch)`':               for some reason it was done with two calls
o19 tputs(^[[C)           readline emitted the 'CUF' control sequence from 
_rl_move_cursor_relative
o1 ^[
o1 [
o1 C
i1 t                      I typed a 't'
o3 tputs[3](^H)           readline emitted 3 backspaces from _rl_backspace
o1 ^H
o1 ^H
o1 ^H
o2 t': I typed this       readline presented this recalled string via 
_rl_output_some_chars

and so on.

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