media: pvrusb2: fix use after free on context disconnection
Upon module load, a kthread is created targeting the pvr2contextthreadfunc function, which may call pvr2contextdestroy and thus call kfree() on the context object. However, that might happen before the usb hubevent handler is able to notify the driver. This patch adds a sanity check before the invalid read reported by syzbot, within the context disconnection call stack.