slirp: check system() success

we shouldn't call W*() macros until we check that fork worked.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

net/slirp.c

@@ -472,7 +472,7 @@ static void slirp_smb_cleanup(SlirpState *s)
     if (s->smb_dir[0] != '\0') {
         snprintf(cmd, sizeof(cmd), "rm -rf %s", s->smb_dir);
         ret = system(cmd);
-        if (!WIFEXITED(ret)) {
+        if (ret == -1 || !WIFEXITED(ret)) {
             qemu_error("'%s' failed.\n", cmd);
         } else if (WEXITSTATUS(ret)) {
             qemu_error("'%s' failed. Error code: %d\n",