test-subprocess: fix segfault without arguments

test-subprocess: fix segfault without arguments

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: Rene Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>