Issue eclipse-ee4j#2016 Fixed NPE on missing CloseReason

- original code was throwing NPE
- handler can be null
  - then we have nothing to handle the reason
- closeReason can be null
  - then we use "locally closed" as a default (may be incorrect)
- even ignored exception should be visible somewhere (logging)

Signed-off-by: David Matějček <dmatej@seznam.cz>

modules/http/src/main/java/org/glassfish/grizzly/http/io/OutputBuffer.java

@@ -40,6 +40,7 @@
 import java.util.logging.Logger;
 
 import org.glassfish.grizzly.Buffer;
+import org.glassfish.grizzly.CloseReason;
 import org.glassfish.grizzly.CompletionHandler;
 import org.glassfish.grizzly.Connection;
 import org.glassfish.grizzly.FileTransfer;
@@ -791,8 +792,14 @@ public void notifyCanWrite(final WriteHandler handler) {
             throw new IllegalStateException("Illegal attempt to set a new handler before the existing handler has been notified.");
         }
 
-        if (!httpContext.getCloseable().isOpen()) {
-            handler.onError(connection.getCloseReason().getCause());
+        if (handler != null && !httpContext.getCloseable().isOpen()) {
+            final CloseReason closeReason = connection.getCloseReason();
+            if (closeReason == null) {
+                LOGGER.log(Level.WARNING, "No close reason set, using default: {0}", CloseReason.LOCALLY_CLOSED_REASON);
+                handler.onError(CloseReason.LOCALLY_CLOSED_REASON.getCause());
+            } else {
+                handler.onError(closeReason.getCause());
+            }
             return;
         }
 
@@ -820,6 +827,7 @@ public void notifyCanWrite(final WriteHandler handler) {
             // have been processed by WriteHandler.onError().
             httpContext.getOutputSink().notifyCanWrite(asyncWriteHandler);
         } catch (Exception ignored) {
+            LOGGER.log(Level.FINE, "Ignoring exception.", ignored);
         }
     }