libssh2: out-of-bounds read

When negotiating a new SSH session with a remote server, one of
libssh2’s functions for doing the key exchange (kex_agree_methods()) was
naively reading data from the incoming packet and using it without doing
sufficient range checks. The SSH_MSG_KEXINIT packet arrives to libssh2
with a set of strings, sent as a series of LENGTH + DATA pairs. libssh2
would go through the list and read the LENGTH field, read the string
following the LENGTH and then advance the pointer LENGTH bytes in memory
and expect to find the next LENGTH + DATA pair there. Then move on until
seven subsequent strings are taken care of. It would naively assume that
the (unsigned 32 bit) LENGTH fields were valid.

This packet arrives in the negotiating phase so the remote server has
not yet been deemed to be a known or trusted party.