channels: fix possible overflow in logging

Fixes clang compiler warning:

"warning: the value of the size argument in 'strncat' is too large,
might lead to a buffer overflow [-Wstrncat-size]"

strncat requires an extra byte for '\0' so dest needs to have a size of
n+1
Merge pull request #2067 from bmiklautz/fix/clog

channels: fix possible overflow in logging