drm/dp/mst: fix buffer overflows in sideband chunk accumulation

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CVE-2026-68278: Undefined Security Weakness

In the Linux kernel, the following vulnerability has been resolved: drm/dp/mst: fix buffer overflows in sideband chunk accumulation drmdpsidebandappendpayload has three related bugs when processing device-provided sideband reply data: 1. Zero-length curchunklen underflow: msglen is a 6-bit field...