Libssh: missing checks for return values for digests

Libssh doesn't properly check if digest operations succeed, which can cause crashes or use uninitialized memory when the system runs low on memory. This results in failed connections but typically doesn't allow attackers to decrypt data.

The libssh crypto abstraction layer fails to validate return values from message digest operations across different backends (CWE-252). Under low-memory conditions, unchecked failures can lead to NULL dereferences, uninitialized memory usage in key derivation, or crashes; while integrity checks prevent successful exploitation for decryption, the flaw enables denial-of-service through connection termination.