libssh2 1.8.1

This release includes the following bugfixes:
 
 o fixed possible integer overflow when reading a specially crafted packet 
   (https://www.libssh2.org/CVE-2019-3855.html)
 o fixed possible integer overflow in userauth_keyboard_interactive with a 
   number of extremely long prompt strings 
   (https://www.libssh2.org/CVE-2019-3863.html)
 o fixed possible integer overflow if the server sent an extremely large number 
   of keyboard prompts (https://www.libssh2.org/CVE-2019-3856.html)
 o fixed possible out of bounds read when processing a specially crafted packet 
   (https://www.libssh2.org/CVE-2019-3861.html)
 o fixed possible integer overflow when receiving a specially crafted exit 
   signal message channel packet (https://www.libssh2.org/CVE-2019-3857.html)
 o fixed possible out of bounds read when receiving a specially crafted exit 
   status message channel packet (https://www.libssh2.org/CVE-2019-3862.html)
 o fixed possible zero byte allocation when reading a specially crafted SFTP 
   packet (https://www.libssh2.org/CVE-2019-3858.html)
 o fixed possible out of bounds reads when processing specially crafted SFTP 
   packets (https://www.libssh2.org/CVE-2019-3860.html)
 o fixed possible out of bounds reads in _libssh2_packet_require(v) 
   (https://www.libssh2.org/CVE-2019-3859.html)

This release would not have looked like this without help, code, reports and
advice from friends like these:

  Chris Coulson, Michael Buckley, Will Cosgrove, Daniel Stenberg
  (4 contributors)