ssh2connection: missing free of antispoof_prompt.

Just noticed that if the session were to abort while that variable
contains some allocated data, it wouldn't be freed.

ssh2connection.c

@@ -310,6 +310,9 @@ static void ssh2_connection_free(PacketProtocolLayer *ppl)
     }
     portfwdmgr_free(s->portfwdmgr);
 
+    if (s->antispoof_prompt)
+        free_prompts(s->antispoof_prompt);
+
     delete_callbacks_for_context(s);
 
     sfree(s);
@@ -999,6 +1002,7 @@ static void ssh2_connection_process_queue(PacketProtocolLayer *ppl)
             s->want_user_input = false;
         }
         free_prompts(s->antispoof_prompt);
+        s->antispoof_prompt = NULL;
     }
 
     /*