mISDN: Fix a use after free in hfcmulti_tx()

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AZL-49008 CVE-2024-42280 affecting package kernel for versions less than 5.15.167.1-1

In the Linux kernel, the following vulnerability has been resolved: mISDN: Fix a use after free in hfcmultitx Don't dereference sp after calling devkfreeskbsp...