CVE-2026-15370: Libssh: libssh: stack buffer overflow in sftp server longname construction

A flaw was found in libssh server-side SFTP directory listing. In processreaddir() in src/sftpserver.c, the server allocates a fixed stack buffer as char longname[MAXLONGNAMELEN] and passes attacker-controlled directory entry names into readdirlongname(). In the pre-patch implementation, readdirlongname() appends metadata and the filename into zlongname with repeated unbounded strcat() calls. Because zlongname points at the caller-owned fixed stack buffer, sufficiently long filenames in an attacker-controlled directory can overflow that stack buffer during SSHFXPREADDIR processing, leading to crashes and possible code execution on the server.