Fix potential integer overflow in Cyrus SASL support for svnserve.

* subversion/svnserve/cyrus_auth.c
  (try_auth): error if we'll overflow the clientinlen argument to sasl
    functions, typecast to silence the compiler warning.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1570434 13f79535-47bb-0310-9956-ffa450edef68