Fix an array-size bug in modmul, and add some tests for it.


git-svn-id: svn://svn.tartarus.org/sgt/putty@9977 cda61777-01e9-0310-a592-d414129be87e
Add some missing bounds checks in signature verification routines.


git-svn-id: svn://svn.tartarus.org/sgt/putty@9978 cda61777-01e9-0310-a592-d414129be87e
Replace some 'sfree' calls of bignums with the proper 'freebn'.


git-svn-id: svn://svn.tartarus.org/sgt/putty@9979 cda61777-01e9-0310-a592-d414129be87e
Verify RSA keys created by rsa2_openssh_createkey.


git-svn-id: svn://svn.tartarus.org/sgt/putty@9980 cda61777-01e9-0310-a592-d414129be87e