make sure we don't run a NULL command

wiz.c

@@ -97,7 +97,7 @@ run(char **cmd)
   waitpid(-1, NULL, WNOHANG);
   if (killchild && cpid)
     kill(cpid, SIGINT);
-  if ((cpid = fork()) == 0) {
+  if (cmd && (cpid = fork()) == 0) {
     dup2(STDERR_FILENO, STDOUT_FILENO);
     setsid();
     execvp(cmd[0], cmd);