graph: fix occasional floating point exception on startup

The mystery of which may never be solved. A change from pprint() to pprint_deb() made all the difference. It occured right before window and OGL init so yeah, a bullet doged. It could have been a lot more serious. Now  I really want to crush that parser bug. Though it doesn't look good. If lua_next() doesn't return 1 there isn't much I can do.