output/csv: fix out-of-bounds array access in process_analog()

output/csv: fix out-of-bounds array access in process_analog()

Make sure to not exceed the ctx->analog_samples[] array bounds. Don't
use the (huge) channel's index in the device's(!) channel list, instead
use the zero-based and dense index into the array of analog samples in
the accumulation buffer, before writing to the external file.

This fixes the segfault reported in bug #1124.