blockdiagram: correct check for non-existing configs

In luajit checking a cdata for being NULL can not be done by negating
it (it is a boxed value, so that evaluates to false). Instead, it must
be explicitely compared against nil.

This problem was previously hidden because block_config_get would
throw an error if a config of the given name didn't exist. Since that
function was updated to just return NULL this problem could occur.

Reported-by: Hamish Guthrie <hamish.guthrie@kistler.com>
Signed-off-by: Markus Klotzbuecher <mk@mkio.de>

lua/blockdiagram.lua

@@ -426,9 +426,11 @@ function system.launch(self, t)
 	       err_exit(1, "invalid node config reference '"..val.."'")
 	    end
 	    local blkcfg = ubx.block_config_get(b, name)
-	    if not blkcfg then
+
+	    if blkcfg==nil then
 	       err_exit(1, "non-existing config '"..blkconf.name.. "."..name.."'")
 	    end
+
 	    ubx.config_assign(blkcfg, _NC[nodecfg])
 	    info("cfg "..green(blkconf.name.."."..blue(name))..
 		    " -> node cfg "..yellow(nodecfg))