mhshow: Avoid SEGV when user's command has two or fewer words.

After argsplit() returned a NULL-terminated vector, the existing NULL
was overwritten with another and the vector index then incremented.
This made the test for whether vec[2] was available out by one resulting
in NULL being used and printed.

This plucking of the third word is a faulty assumption as argsplit()
doesn't always return ["sh", "-c", ...], but add a new test script for
it to avoid regression and so that the script can be expanded with more
related tests.

Fixes 7559e1ebf.