Fix -Wformat issue with zero-length format string passed to err(3)

MFC after:	3 days
Tested with:	clang, gcc 4.2.1, gcc 4.9
Sponsored by:	Dell EMC Isilon

tests/sys/kern/execve/execve_helper.c

@@ -50,5 +50,5 @@ main(int argc, char **argv)
 	}
 
 	execve(argv[1], &argv[1], NULL);
-	err(1, "");
+	err(1, "%s", "");
 }