skill: Always NULL-terminate argv.

The memmove() itself does not move the NULL-terminator, because nargs is
decremented first. Copy how skill_sig_option() does it: decrement nargs
last, and remove the "if (nargs - i)" (we are in "while (i < nargs)").

skill.c

@@ -542,10 +542,9 @@ static int snice_prio_option(int *argc, char **argv)
 			if (prio < INT_MIN || INT_MAX < prio)
 				xerrx(EXIT_FAILURE,
 				     _("priority %lu out of range"), prio);
-			nargs--;
-			if (nargs - i)
 			memmove(argv + i, argv + i + 1,
 				sizeof(char *) * (nargs - i));
+			nargs--;
 		} else
 			i++;
 	}