control: HSFETCH command fails to validate v2 addresses

In handle_control_hsfetch():

  } else if (strcmpstart(arg1, v2_str) == 0 &&
             rend_valid_descriptor_id(arg1 + v2_str_len) &&
             base32_decode(digest, sizeof(digest), arg1 + v2_str_len,
                           REND_DESC_ID_V2_LEN_BASE32) ==
                REND_DESC_ID_V2_LEN_BASE32) {

The above snippet is how we validate the v2 address for the HSFETCH command. The base32_decode() function returns the number of bytes decoded and thus it should returns on success sizeof(digest), not the total length of the base32 address (20 vs 32).

Issue was introduced in commit a517daa56f5848d25ba79617a1a7b82ed2b0a7c0 meaning since 0.4.1.1-alpha (ticket legacy/trac#28913 (moved)).

I noticed this because I recently updated the bad HSDirscanner Tor to use our latest and it broke the scanner.