Short circuit SASL auth when no mechanisms are available

If the SASL config does not have any mechanisms we currently
just report an empty list to the client which will then
fail to identify a usable mechanism. This is a server config
error, so we should fail immediately on the server side.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

src/rpc/virnetsaslcontext.c

@@ -390,6 +390,12 @@ char *virNetSASLSessionListMechanisms(virNetSASLSessionPtr sasl)
                        err, sasl_errdetail(sasl->conn));
         goto cleanup;
     }
+    VIR_DEBUG("SASL mechanism list is '%s'", mechlist);
+    if (STREQ(mechlist, "")) {
+        virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                       _("no SASL mechanisms are available"));
+        goto cleanup;
+    }
     ignore_value(VIR_STRDUP(ret, mechlist));
 
  cleanup: