iio: adc: xilinx-ams: fix out-of-bounds channel lookup in event handling

In the Linux kernel, the following vulnerability has been resolved: iio: adc: xilinx-ams: fix out-of-bounds channel lookup in event handling ams_event_to_channel() may return a pointer past the end of dev->channels when no matching scan_index is found. This can lead to invalid memory access in ams_handle_event(). Add a bounds check in ams_event_to_channel() and return NULL when no channel is found. Also guard the caller to safely handle this case.