Avoid illegal read off end of an array in prune_v2_cipher_list

This function is supposed to construct a list of all the ciphers in
the "v2 link protocol cipher list" that are supported by Tor's
openssl.  It does this by invoking ssl23_get_cipher_by_char on each
two-byte ciphersuite ID to see which ones give a match.  But when
ssl23_get_cipher_by_char cannot find a match for a two-byte SSL3/TLS
ciphersuite ID, it checks to see whether it has a match for a
three-byte SSL2 ciphersuite ID.  This was causing a read off the end
of the 'cipherid' array.

This was probably harmless in practice, but we shouldn't be having
any uninitialized reads.

(Using ssl23_get_cipher_by_char in this way is a kludge, but then
again the entire existence of the v2 link protocol is kind of a
kludge.  Once Tor 0.2.2 clients are all gone, we can drop this code
entirely.)

Found by starlight. Fix on 0.2.4.8-alpha. Fixes bug 12227.