QemuOpts: check NULL input for qemu_opts_del

To simplify later using of qemu_opts_del, accept NULL input.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Leandro Dorileo <l@dorileo.org>
Signed-off-by: Chunyan Liu <cyliu@suse.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/util/qemu-option.c b/util/qemu-option.c
index 6acfb0e..40e1ff3 100644
--- a/util/qemu-option.c
+++ b/util/qemu-option.c
@@ -1011,6 +1011,10 @@
 {
     QemuOpt *opt;
 
+    if (opts == NULL) {
+        return;
+    }
+
     for (;;) {
         opt = QTAILQ_FIRST(&opts->head);
         if (opt == NULL)