fix: strncpy equals destination size warning

fix: strncpy equals destination size warning

Some versions of GCC when called with -Wstringop-truncation will warn
when doing a copy of the same size as the destination buffer with
strncpy :

  ‘strncpy’ specified bound 256 equals destination size [-Werror=stringop-truncation]

Since we unconditionally write '\0' in the last byte, reduce the copy
size by one.

Change-Id: Idb907c9550817a06fc0dffc489740f63d440e7d4
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>