egl-helpers: fix possible resource leak

CID 1352419, using g_strdup_printf instead of asprintf.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1463047028-123868-2-git-send-email-arei.gonglei@huawei.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
