xen-netfront: handle NULL returned by xdp_convert_buff_to_frame()

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                        "drivers/net/xen-netfront.c"
                    ],
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{
                    "product": "Linux",
                    "vendor": "Linux",
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                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "drivers/net/xen-netfront.c"
                    ],
                    "versions": [
                        {
                            "version": "5.9",
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                        },
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                }