can not comint-interrupt-subjob

[Ilya N. Golubev]

Reproducing in 21.4 as follows.

Inside `shell' run `bash-3.0-31'.

Run a background job inside it.  Any interactive graphical program
will do, or even

perl -e 'use POSIX;pause;'&

Run a foreground job in `bash'.  `cat' will do.

Terminate background job - by command in itself, or by `kill' from
another shell, so that `bash' outputs job status message.

`comint-interrupt-subjob' in shell buffer outputs an (elisp error)
message:

Wrong type argument: integer-or-marker-p, nil