nofork comsub in PS1 + ignoreeof

[Grisha Levit]

When a nofork command substitution is in PS1 and ignoreeof is enabled,
(sufficiently repeated) EOF doesn't actually cause the shell to exit:

$ PS1='${ :;}$ ' IGNOREEOF=1 bash --norc
$ ^D
Use "exit" to leave the shell.
$ ^D
Use "exit" to leave the shell.
...