Add NPE protection

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1607931 13f79535-47bb-0310-9956-ffa450edef68
Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=56677
Ensure that HttpServletRequest.getServletContext() returns the correct value during a cross-context dispatch. 

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1607934 13f79535-47bb-0310-9956-ffa450edef68