dialogue conditions: make is_safe work for tiles without monster groups

The dialogue condition is_safe checks whether the overmap terrain
tile is_safe(), and that in turn checks if there are any unsafe
monster groups on the tile.  If a tile has no monster groups (and many
don't), the tile is safe.  This results is some weirdness when an
avatar and an NPC are in hand-to-hand combat with zombies and the NPC
is perfectly willing to train the avatar in lockpicking.

Add a new "is_safe()" function to talker, talker_npc, and npc, and
have the npc version return false if the NPC's ai_cache.total_danger
is greater than 0, and then return the union of the tile's is_safe()
and the talker's is_safe().  That seems to resolve the issue.

src/condition.cpp

@@ -695,7 +695,7 @@ template<class T>
 void conditional_t<T>::set_at_safe_space()
 {
     condition = []( const T & d ) {
-        return overmap_buffer.is_safe( d.beta->global_omt_location() );
+        return overmap_buffer.is_safe( d.beta->global_omt_location() ) && d.beta->is_safe();
     };
 }
 

src/npc.h

@@ -1040,6 +1040,7 @@ class npc : public player
         float evaluate_enemy( const Creature &target ) const;
 
         void assess_danger();
+        bool is_safe() const;
         // Functions which choose an action for a particular goal
         npc_action method_of_fleeing();
         npc_action method_of_attack();

src/npcmove.cpp

@@ -644,6 +644,11 @@ void npc::assess_danger()
     ai_cache.danger_assessment = assessment;
 }
 
+bool npc::is_safe() const
+{
+    return ai_cache.total_danger <= 0;
+}
+
 float npc::character_danger( const Character &uc ) const
 {
     // TODO: Remove this when possible

src/talker.h

@@ -311,5 +311,8 @@ class talker
         virtual void add_opinion( int /*trust*/, int /*fear*/, int /*value*/, int /*anger*/,
                                   int /*debt*/ ) {}
         virtual void set_first_topic( const std::string & ) {}
+        virtual bool is_safe() const {
+            return true;
+        }
 };
 #endif // CATA_SRC_TALKER_H

src/talker_npc.cpp

@@ -888,3 +888,8 @@ void talker_npc::set_first_topic( const std::string &chat_topic )
 {
     me_npc->chatbin.first_topic = chat_topic;
 }
+
+bool talker_npc::is_safe() const
+{
+    return me_npc->is_safe();
+}

src/talker_npc.h

@@ -108,6 +108,7 @@ class talker_npc : public talker_character
         void add_opinion( int trust, int fear, int value, int anger, int debt ) override;
         bool enslave_mind() override;
         void set_first_topic( const std::string &chat_topic ) override;
+        bool is_safe() const override;
 
     protected:
         npc *me_npc;