Fix: sessiond: g++ reports shm_path truncation issue

Fix: sessiond: g++ reports shm_path truncation issue

On the CI, g++ reports:
  char* strncpy(char*, const char*, size_t)' specified bound 4096 equals destination size [-Wstringop-truncation]

The shm_path provided by liblttng-ctl is truncated to PATH_MAX
regardless of its contents.

LTTN_ERR_INVALID is returned if the shm_path is not null terminated.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ie369f2ee368c70e2b3b41c45a69da7e6d9fa45f8