Fix system.datestr(0) returning garbage

Actually use the return value of datestr()

Fixes issue reported by Deuce, regression introduced in commit 490e86fa

src/sbbs3/js_system.c

@@ -1293,8 +1293,7 @@ js_datestr(JSContext *cx, uintN argc, jsval *arglist)
 		}
 		JS_ValueToECMAUint32(cx,argv[0],(uint32_t*)&t);
 	}
-	datestr(sys->cfg,t,str);
-	if((js_str = JS_NewStringCopyZ(cx, str))==NULL)
+	if((js_str = JS_NewStringCopyZ(cx, datestr(sys->cfg, t, str))==NULL)
 		return(JS_FALSE);
 
 	JS_SET_RVAL(cx, arglist, STRING_TO_JSVAL(js_str));