fix: fixed edge case where session not set and uncaught exception occurs (#780)

Author: titanism

lib/pop3/connection.js

@@ -998,6 +998,12 @@ class POP3Connection extends EventEmitter {
                     username,
                     'PLAIN'
                 );
+
+                if (!this.session) {
+                    // already closed, do nothing
+                    return;
+                }
+                    
                 this.session.user = response.user;
 
                 this.openMailbox(err => {