write_sensu: use sstrncpy

Fixes:
  CC       src/write_sensu.lo
src/write_sensu.c: In function ‘replace_str’:
src/write_sensu.c:630:3: warning: ‘strncpy’ output truncated before terminating nul copying as many bytes from a string as its length [-Wstringop-truncation]
   strncpy(r, p, strlen(p));
   ^~~~~~~~~~~~~~~~~~~~~~~~
  CCLD     write_sensu.la