transport.c: Fix crash with delayed compression (#443)

Files: transport.c

Notes:
Fixes crash with delayed compression option using Bitvise server.

Contributor:
Zenju

src/transport.c

@@ -765,7 +765,7 @@ int _libssh2_transport_send(LIBSSH2_SESSION *session,
         ((session->state & LIBSSH2_STATE_AUTHENTICATED) ||
          session->local.comp->use_in_auth);
 
-    if(encrypted && compressed) {
+    if(encrypted && compressed && session->local.comp_abstract) {
         /* the idea here is that these function must fail if the output gets
            larger than what fits in the assigned buffer so thus they don't
            check the input size as we don't know how much it compresses */