mg(1): Make sure we don't deref NULL ptr in skipwhite()

[Joachim Nilsson]

--- a/src/interpreter.c
+++ b/src/interpreter.c
@@ -139,7 +139,10 @@ multiarg(char *funstr)
                return (dobeep_msgs("Command takes no arguments: ", cmdp));

        /* now find the first argument */
-       p = fendp + 1;
+       if (fendp)
+               p = fendp + 1;
+       else
+               p = "";
        p = skipwhite(p);
        if (strlcpy(argbuf, p, sizeof(argbuf)) >= sizeof(argbuf))
                return (dobeep_msg("strlcpy error"));