bin: copy: fix segfault for one argument

The code assumes both SRC and DST is passed, here we ensure this
early.

Prior to this patch:
root@host:~$ copy foo
Segmentation fault (core dumped)

Signed-off-by: Richard Alpe <richard@bit42.se>

src/bin/copy.c

@@ -418,7 +418,7 @@ int main(int argc, char *argv[])
 	if (timeout < 0)
 		timeout = 120;
 
-	if (optind >= argc)
+	if (argc - optind != 2)
 		return usage(1);
 
 	src = argv[optind++];