Fix gcc warning -Wsizeof-pointer-memaccess

iodined.c: In function ‘write_dns_nameenc’:
iodined.c:2030:23: attention : argument to ‘sizeof’ in ‘memset’ call is the same
expression as the destination; did you mean to provide an explicit length? [-Wsizeof-pointer-memaccess]
  memset(buf, 0, sizeof(buf));

sizeof buf will just give the size of the pointer, while buflen will clean the whole
memory.