Negotiation doesn't fail if one of the I/O sides requires non-existing multipipe I/O

[dspinellis]

The following example (see issue #63) core-dumps. It should exit with an error, because the output of cut isn't pasted to cat.

dgsh -c 'echo "foo bar baz" | cut -d" " -f 1,3 --multistream'

The following, correctly, works fine

dgsh -c 'echo "foo bar baz" | cut -d" " -f 1,3 --multistream | cat'