Call afterProperties set in LogFileMvcEndpoint

Call afterPropertiesSet on the delegate `ResourceHttpRequestHandler`
to prevent an NPE. This change is required for compatibility with
Spring Framework 4.3 following SPR-13834.

Closes gh-6592

Author: philwebb

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/mvc/LogFileMvcEndpoint.java

@@ -147,6 +147,12 @@ private static class Handler extends ResourceHttpRequestHandler {
 
 		Handler(Resource resource) {
 			this.resource = resource;
+			try {
+				afterPropertiesSet();
+			}
+			catch (Exception ex) {
+				throw new IllegalStateException(ex);
+			}
 		}
 
 		@Override