QemuOpts: check NULL input for qemu_opts_del

To simplify later using of qemu_opts_del, accept NULL input.
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NLeandro Dorileo <l@dorileo.org>
Signed-off-by: NChunyan Liu <cyliu@suse.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

util/qemu-option.c

@@ -1011,6 +1011,10 @@ void qemu_opts_del(QemuOpts *opts)
 {
     QemuOpt *opt;
 
+    if (opts == NULL) {
+        return;
+    }
+
     for (;;) {
         opt = QTAILQ_FIRST(&opts->head);
         if (opt == NULL)