Check if we even have a parameter before checking its value.  Running
this command without any arguments may not make a lot of sense, but
reacting with a segmentation fault is unduly harsh.

While we're at it, avoid casting argv by declaring it const right away.
Signed-off-by: NRene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>