Bug#37300558 segmentation fault in Ndb_cluster_connection Dtor

Problem:
Ndb_cluster_connection destructor calls g_eventLogger::stopAsync()
in order to release the buffers used by the async log mechanism as
well as to stop the threads responsible for the async logging.
Problem is that, if g_eventLogger object is deleted before the
destructor of the Ndb_cluster_connection is called, program crash
because it is trying to use a method on a null object.

This can happen in two different ways:

1- Api programs deletes the logger object before deleting the
   Ndb_cluster_connection
2- ndb_end() is called before Ndb_cluster_connection is deleted.

Solution:
In cluster_connection Destructor do not call stopAsync() when
g_eventLogger object is NULL.
A warning is also added to inform API users that g_eventLogger
is *wrongly* deleted before cluster_connection Dtor is called.

Change-Id: I57ff648949a60b45841e8b568030259f1540180b

Author: vinc13e

storage/ndb/src/ndbapi/ndb_cluster_connection.cpp

@@ -613,7 +613,14 @@ Ndb_cluster_connection_impl::~Ndb_cluster_connection_impl() {
     NdbMutex_Destroy(ndb_print_state_mutex);
     ndb_print_state_mutex = nullptr;
 #endif
-    g_eventLogger->stopAsync();
+    if (g_eventLogger != nullptr) {
+      g_eventLogger->stopAsync();
+    } else {
+      [[maybe_unused]] int ret =
+          fprintf(stderr,
+                  "WARNING: g_eventLogger object is "
+                  "deleted. ndb_end() called too soon?\n");
+    }
   }
   NdbMutex_Unlock(g_ndb_connection_mutex);