ssh: Fix adding an ed25519 key with a zero length comment.
* agent/command-ssh.c (sexp_key_construct): Do not put an empty string into an S-expression. (stream_read_string): Do not not try to a read a zero length block.
Actually we could handles this different by not putting a comment tag
into the s-expression, however this requires more code and at other
places we already return "(none)" instead of an empty comment.
The second fix is more or less a cosmetic thing to get better error
messages in case the underlying read system call returns an error.
- GnuPG-bug-id: T5794