REDHAT-BUG-2498177: Medium severity libssh SFTP server vulnerability

A flaw was found in libssh server-side SFTP request handling. In sftpmakeclientmessage() and the downstream processread() path in src/sftpserver.c, the implementation accepts a client-controlled SSHFXPREAD length without enforcing a reasonable upper bound. The parsed len field later drives memory allocation in the SFTP server read path, so a remote authenticated client can request an excessively large read length and force the server to allocate excessive memory. Repeated requests can exhaust available memory and lead to denial of service.