HID: prodikeys: Check presence of pm->input_ep82

In the Linux kernel, the following vulnerability has been resolved: HID: prodikeys: Check presence of pm->input_ep82 Fake USB devices can send their own report descriptors for which the input_mapping() hook does not get called. In this case, pm->input_ep82 stays NULL, which leads to a crash later. This does not happen with the real device, but can be provoked by imposing as one.