wifi: mt76: mt7921/mt7925: fix NULL dereference in CSA beacon

In the Linux kernel, the following vulnerability has been resolved: wifi: mt76: mt7921/mt7925: fix NULL dereference in CSA beacon This patch is based on a BUG as reported by Bongani Hlope at https://lore.kernel.org/all/20260502125824.425d7159@bongani-mini.home.org.za/ When a channel-switch announcement (CSA) beacon is received, cfg80211 queues a wiphy work item that eventually calls mt7921_channel_switch_rx_beacon(). If the station disconnects (or the channel context is otherwise torn down) between the time the work is queued and the time it runs, the driver's dev->new_ctx pointer can already have been cleared to NULL. mt7921_channel_switch_rx_beacon() then dereferences new_ctx unconditionally, triggering a NULL pointer dereference at address 0x0: BUG: kernel NULL pointer dereference, address: 0000000000000000 RIP: 0010:mt7921_channel_switch_rx_beacon+0x1f/0x100 [mt7921_common] The same missing guard exists in mt7925_channel_switch_rx_beacon(), which shares the same code pattern introduced by the same commit. Add an early-return NULL check for dev->new_ctx in both mt7921_channel_switch_rx_beacon() and mt7925_channel_switch_rx_beacon(). When new_ctx is NULL there is no pending channel switch to process, so returning immediately is the correct and safe action. Oops-Analysis: http://oops.fenrus.org/reports/lkml/20260502125824.425d7159@bongani-mini.home.org.za/report.html