dropbear: 2016.73 -> 2016.74

Security fixes:
- Message printout was vulnerable to format string injection
- dropbearconvert import of OpenSSH keys could run arbitrary code
as the local dropbearconvert user when parsing malicious key
files
- dbclient could run arbitrary code as the local dbclient user if
particular -m or -c arguments are provided
- dbclient or dropbear server could expose process memory to the
running user if compiled with DEBUG_TRACE and running with -v

Fixes:
- Fix port forwarding failure when connecting to domains that have
both IPv4 and IPv6 addresses. The bug was introduced in 2015.68
- Fix 100% CPU use while waiting for rekey to complete

pkgs/tools/networking/dropbear/default.nix

···
 sftpPath ? "/var/run/current-system/sw/libexec/sftp-server" }:
 
 stdenv.mkDerivation rec {
-  name = "dropbear-2016.73";
+  name = "dropbear-2016.74";
 
   src = fetchurl {
     url = "http://matt.ucc.asn.au/dropbear/releases/${name}.tar.bz2";
-    sha256 = "1mzg18jss1bsmcnn88zv7kv5yj01hzimndnd5636hfq9kgva8qaw";
+    sha256 = "14c8f4gzixf0j9fkx68jgl85q7b05852kk0vf09gi6h0xmafl817";
   };
 
   dontDisableStatic = enableStatic;