site: transport peers: fix incorrect stride when debug output enabled

When there are multiple peer addresses, attempts to copy them from one
table of peers addresses to another with transport_peers_copy will go
wrong because the stride argument to transport_peers_debug is wrong -
we take sizeof() the pointer rather than of the array element.

This will typically cause a segfault if it happens, but the bug can
only be triggered if LOG_PEER_ADDRS debugging is enabled.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>