Fix image resource leak

[reuteran]

Caused by: java.io.FileNotFoundException: /prog/apache-tomcat-7.0.57/work/Catalina/localhost/_/eclipse/configuration/org.eclipse.osgi/51/data/rwtcontext_267262024_1484531491/rwt-resources/resources/i
mages/valid_icon.png (Zu viele offene Dateien)
        at java.io.FileInputStream.open(Native Method)
        at java.io.FileInputStream.<init>(FileInputStream.java:146)
        at org.eclipse.rap.rwt.internal.resources.ResourceManagerImpl.getRegisteredContent(ResourceManagerImpl.java:116)
        ... 80 more
Exception occurred
java.lang.RuntimeException: Failed to register resource: generated/5a393821.png