qdev: Fix crash on -device '?=x'

Spotted by Coverity.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

hw/qdev.c

@@ -186,7 +186,7 @@ int qdev_device_help(QemuOpts *opts)
         return 1;
     }
 
-    if (!qemu_opt_get(opts, "?")) {
+    if (!driver || !qemu_opt_get(opts, "?")) {
         return 0;
     }