g_child_watch_source_new: POSIX pid must be positive

If we used a non-positive pid, we'd call waitpid(that_pid, ...)
which is exactly the situation this function can't deal with.

On Windows, GPid is a HANDLE (pointer), so I don't think the same thing
applies.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=723743
Reviewed-by: Ryan Lortie