qemu: Fix possible NULL deref in qemuDomainSaveImageStartVM

Commit 075523438 added a direct reference to @cookie even though
it may be NULL as shown by a comment a few lines previous - so add
the check here as well.

Found by Coverity

Signed-off-by: John Ferlan <jferlan@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

src/qemu/qemu_driver.c

@@ -6831,7 +6831,7 @@ qemuDomainSaveImageStartVM(virConnectPtr conn,
         qemuDomainFixupCPUs(vm, &cookie->cpu) < 0)
         goto cleanup;
 
-    if (!cookie->slirpHelper)
+    if (cookie && !cookie->slirpHelper)
         priv->disableSlirp = true;
 
     if (qemuProcessStart(conn, driver, vm, cookie ? cookie->cpu : NULL,