In function ‘wadj_cfg_req.isra.1’,
    inlined from ‘vmod_random_add_backend’ at
vmod_weightadjust.c:291:14:
vmod_weightadjust.c:170:10: error: ‘strncpy’ output truncated before
terminating nul copying as many bytes from a string as its length
[-Werror=stringop-truncation]

gcc does not understand that we are adding a NUL ourselves. Avoid this
warning by a differnt but equally safe approach.

Reported by volker27@gmx.at, thank you

to be consistent with remove_backend

As of cf3c9d9213c12e3c58a391b068398f8fd1d1c17c

Fixes #5