hs: Maybe don't use REND_DESC_ID_V2_LEN_BASE32 as the length for a base32 relay digest id

Function hs_lookup_last_hid_serv_request() defines an HSDir identity digest in base32 as:

  char hsdir_id_base32[REND_DESC_ID_V2_LEN_BASE32 + 1];

Although the length is correct, semantically this is bad to use the base32 descriptor ID length for this.