Patch by David Nowotny
In make_matrix (channelmix-ops.c), the matrix-filling loop at the done:
label allows the index i (and j in the inner loop) to grow beyond MAX_CHANNELS
when dst_paired/src_paired has sparse bits set. In that case the continue
fires for most values of i < CHANNEL_BITS, so i advances much faster than ic,
and matrix[i][j] reads off the end of the stack-allocated array.
Add bounds guards to both loop conditions so i and j cannot exceed
MAX_CHANNELS.
Fixes#5176