Properly handle 0 being passed as a signal number

At present, "docker kill -s 0 $CONTAINER" is a valid, accepted command, which sends SIGKILL - which doesn't really make much sense, as 0 is not a valid signal, and SIGKILL is not signal 0.

This patch adds another condition to the parsing of the signal number to ensure that not only is it a valid integer from 0 to 31, it is also nonzero.