cmirrord: fix stack smashing

cmirrord: fix stack smashing

With clusters larger than 3 nodes, the 32-byte debug buffer in
cpg_join_callback() is too small to contain all the node IDs, because
32-bit identifiers are generally rendered in 10 decimal digits.  No fixed
size is good in all cases, but this is conditionally logged debug info,
so we can simply truncate it.  Double the size, nevertheless.