qemuMonitorJSONSendKey: Avoid double free

After successful @cmd construction the memory where @keys points to is
part of @cmd. Avoid double freeing it.
(cherry picked from commit 3e8343e1)

src/qemu/qemu_monitor_json.c

@@ -3440,6 +3440,9 @@ int qemuMonitorJSONSendKey(qemuMonitorPtr mon,
     if (!cmd)
         goto cleanup;
 
+    /* @keys is part of @cmd now. Avoid double free */
+    keys = NULL;
+
     if ((ret = qemuMonitorJSONCommand(mon, cmd, &reply)) < 0)
         goto cleanup;