red-stream: Avoid infinite loop on sasl_encode/decode failure

These functions do not set errno so it is possible that errno has a
stale value which happens to be EAGAIN.
This would cause an infinite loop in functions like red_stream_write_all
(or potentially using the event loop).

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Acked-by: Christophe Fergeau <cfergeau@redhat.com>

server/red-stream.c

@@ -596,6 +596,7 @@ static ssize_t red_stream_sasl_write(RedStream *s, const void *buf, size_t nbyte
                           &s->priv->sasl.encodedLength);
         if (err != SASL_OK) {
             spice_warning("sasl_encode error: %d", err);
+            errno = EIO;
             return -1;
         }
 
@@ -657,6 +658,7 @@ static ssize_t red_stream_sasl_read(RedStream *s, uint8_t *buf, size_t nbyte)
                       &decoded, &decodedlen);
     if (err != SASL_OK) {
         spice_warning("sasl_decode error: %d", err);
+        errno = EIO;
         return -1;
     }