try to fix RrdGraph GIF exception

apps/routerconsole/java/src/net/i2p/router/web/StatSummarizer.java

@@ -334,6 +334,8 @@ public class StatSummarizer implements Runnable {
             //System.out.println("*****************\nData: \n" + _listener.getData().dump());
             def.setWidth(width);
             def.setHeight(height);
+            def.setImageFormat("PNG");
+            def.setLazy(true);
 
             RrdGraph graph = new RrdGraph(def);
             //System.out.println("Graph created");

apps/routerconsole/java/src/net/i2p/router/web/SummaryRenderer.java

@@ -164,6 +164,8 @@ class SummaryRenderer {
             //System.out.println("*****************\nData: \n" + _listener.getData().dump());
             def.setWidth(width);
             def.setHeight(height);
+            def.setImageFormat("PNG");
+            def.setLazy(true);
 
             RrdGraph graph = new RrdGraph(def);
             int totalWidth = graph.getRrdGraphInfo().getWidth();