Complete implementation of error logging when no session pointer in DCB.

server/core/poll.c

@@ -1132,7 +1132,13 @@ poll_dcb_session_check(DCB *dcb)
     }
     else
     {
-        LOGIF;
+        LOGIF(LE, (skygw_log_write_flush(
+            LOGFILE_ERROR,
+            "%lu [%s] The dcb %p that was about to be processed does not "
+            "have a non-null session pointer "
+            pthread_self(),
+            __func__,
+            dcb)));
         return false;
     }
 }