can not piggyback comint commands

[Ilya N. Golubev]

In stable version of `comint.el' could type next command to inferior
process while the previous one executes and outputs data, and type RET
/ `comint-send-input'.  When the previous command completed, the next
command would be passed to process, and it would start executing it.
With `comint.el' revision 1.18 of 2006/07/11 23:45:46 +0 can no longer
do that.

Reproducing as follows.  In `shell' mode buffer send command:

(while:;do echo .;sleep 1;done)

Then run `end-of-buffer', append the next command:

(while:;do echo ,;sleep 1;done)

Type RET.

Kill the shell child process printing `.'.

With stable `comint' shell would start `,'.  With latest one it does
nothing, have to type one more RET to start the command typing `,'.

Piggybacking commands like that is extremely useful with lengthy jobs
like compilation of large packages that for various reasons must be
done in sequence, not in the same time.