ssh2: fix positive mpint packing. Closes #2338

CHANGELOG

@@ -1,5 +1,8 @@
 #Nmap Changelog ($Id$); -*-text-*-
 
+o [GH#2338][NSE] Fix mpint packing in ssh2 library, which was causing OpenSSH
+  errors like "ssh_dispatch_run_fatal: bignum is negative" [Sami Loone]
+
 o Additional Python 3 update fixes by Sam James, Daniel Miller. Special thanks
   to those who opened Python 3-related issues and pull requests: Eli Schwartz, Romain
   Leonard, Varunram Ganesh, Pavel Zhukov, Carey Balboa, Hasan Aliyev, and others.

nselib/ssh2.lua

@@ -57,7 +57,7 @@ transport.pack_mpint = function( bn )
   local bytes, packed
   bytes = bn:num_bytes()
   packed = bn:tobin()
-  if bytes % 8 == 0 then
+  if bn:num_bits() % 8 == 0 then
     bytes = bytes + 1
     packed = '\0' .. packed
   end