bs_sheepdog.c: fix an unsafe strncpy() call

The strncpy() call doesn't take a null terminator into account. On
some environment, it conflicts with __builtin___strncpy_chk() and gcc
warns.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>