Remove a return path from sshcom_write() which was both unreachable

Remove a return path from sshcom_write() which was both unreachable
(it would trigger if !type==RSA and !type==DSA, but one of those must
have been true to get there in the first place) and erroneous (it
would return NULL without going through the cleanup code). Since the
code's internal structure guarantees that path isn't reached, replace
it with an assert.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9924 cda61777-01e9-0310-a592-d414129be87e