We added a NUL one line down, but make the compiler happy...

In function ‘strncpy’,
    inlined from ‘wadj_cfg_req’ at vmod_weightadjust.c:179:10,
    inlined from ‘vmod_random_add_backend’ at vmod_weightadjust.c:300:14:
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:95:10: error: ‘__builtin_strncpy’ output truncated before terminating nul copying as many bytes from a string as its length [-Werror=stringop-truncation]
   95 |   return __builtin___strncpy_chk (__dest, __src, __len,
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   96 |                                   __glibc_objsize (__dest));
      |                                   ~~~~~~~~~~~~~~~~~~~~~~~~~
vmod_weightadjust.c: In function ‘vmod_random_add_backend’:
vmod_weightadjust.c:169:36: note: length computed here
  169 |         const int reql = request ? strlen(request) + 1 : 0;
      |                                    ^~~~~~~~~~~~~~~

to be consistent with remove_backend

As of cf3c9d9213c12e3c58a391b068398f8fd1d1c17c

Fixes #5