CTLESC and CTLNUL removed in certain contexts

[Grisha Levit]

(this is maybe too edge-case to be worth looking at)

A couple circumstances cause repeated instances of \001 and any
instances of \177 to be removed from as string:

$ a=$'<\001\001><\177\177>'
$ z=$(compgen -W "$a"); echo "${z@A}"
z=$'<\001><>'
$ declare -A A=(); printf -v "A[$a]" X; declare -p A
declare -A A=([$'<\001><>']="X" )

The latter is resolved if assoc_expand_once is turned on.