monitor: Fix missing err = NULL in client_migrate_info()

When SPICE isn't used, we either fail an assertion in error_set(),
or leak an error object.  Broken in commit b25d81ba.

Cc: Markus Armbruster <armbru@redhat.com>
Signed-off-by: NGonglei <arei.gonglei@huawei.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>

monitor.c

@@ -1095,12 +1095,13 @@ static int client_migrate_info(Monitor *mon, const QDict *qdict,
     const char *subject  = qdict_get_try_str(qdict, "cert-subject");
     int port             = qdict_get_try_int(qdict, "port", -1);
     int tls_port         = qdict_get_try_int(qdict, "tls-port", -1);
-    Error *err;
+    Error *err = NULL;
     int ret;
 
     if (strcmp(protocol, "spice") == 0) {
         if (!qemu_using_spice(&err)) {
             qerror_report_err(err);
+            error_free(err);
             return -1;
         }