libssh2: version bump/ CVE fixes

libssh2: version bump/ CVE fixes

- CVE-2019-3855
  Possible integer overflow in transport read allows out-of-bounds write

- CVE-2019-3856
  Possible integer overflow in keyboard interactive handling allows
  out-of-bounds write

- CVE-2019-3857
  Possible integer overflow leading to zero-byte allocation and out-of-bounds
  write

- CVE-2019-3858
  Possible zero-byte allocation leading to an out-of-bounds read

- CVE-2019-3859
  Out-of-bounds reads with specially crafted payloads due to unchecked use of
  `_libssh2_packet_require` and `_libssh2_packet_requirev`

- CVE-2019-3860
  Out-of-bounds reads with specially crafted SFTP packets

- CVE-2019-3861
  Out-of-bounds reads with specially crafted SSH packets

- CVE-2019-3862
  Out-of-bounds memory comparison

- CVE-2019-3863
  Integer overflow in user authenicate keyboard interactive allows
  out-of-bounds writes

Signed-off-by: Sebastian Kemper <redacted>