Suppress npcs offer to train if the npc can not train any skill

The actual problem seems that the talk goes back to TALK_MISSION_SUCCESS, but without a mission, as selecting TALK_TRAIN also runs `talk_function::clear_mission`.
The talk goes back to TALK_MISSION_SUCCESS, but there is no mission anymore.

I had this backtrace for the crash:
```
0  0x080654d6 in gen_responses (topic=TALK_MISSION_SUCCESS, p=0x8a39660) at src/npctalk.cpp:971
1  0x0806e5bd in dialogue::opt (this=0xbf8dfb6c, topic=TALK_MISSION_SUCCESS) at src/npctalk.cpp:1963
2  0x08060f57 in npc::talk_to_u (this=0x8a39660) at src/npctalk.cpp:468
3  0x083dc5e6 in game::chat (this=0xb7057008) at src/game.cpp:11139
4  0x0839ea10 in game::handle_action (this=0xb7057008) at src/game.cpp:2541
5  0x08394402 in game::do_turn (this=0xb7057008) at src/game.cpp:968
6  0x080a701f in main (argc=0, argv=0xbf8e04f8) at src/main.cpp:147
```