media: pvrusb2: fix use after free on context disconnection

In the Linux kernel, the following vulnerability has been resolved:

media: pvrusb2: fix use after free on context disconnection

Upon module load, a kthread is created targeting the

pvr2_context_thread_func function, which may call pvr2_context_destroy

and thus call kfree() on the context object. However, that might happen

before the usb hub_event 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.