_arguments bug

[Oliver Kiddle <opk@xxxxxxxxxxxxx>]

_arguments doesn't work if the first argument doesn't exclude any other
arguments:

_foo() {
  _arguments -C '-m[m]:m'
}
compdef _foo foo
foo -<tab>

This hangs until I press Ctrl-C. If I use '(-x)-m[m]:m' instead, it will
work.

This bug was not caused by any of this mornings patches as I was having
problems last week: it was only today that I tried to trace a cause.

Oliver