Fixed Debian bug #729276 - buffer overflow.


git-svn-id: svn://svn.code.sf.net/p/staden/code/io_lib@3529 ba6f2710-ca9f-427c-89d3-5bf716dbc8e1
Added an implementation of cram_external_encode(), although it's only
for experimental purposes as it's not used in vanilla code (we fill
out the core blocks in-situ rather than via the function pointer).


git-svn-id: svn://svn.code.sf.net/p/staden/code/io_lib@3530 ba6f2710-ca9f-427c-89d3-5bf716dbc8e1