Bugfix: Prevent segfault for npc get_restock_interval

* Prevents segfault that previously happened when choosing the talk option "Find anything good?" when talking to scrap trader.

Previous problem was caused by:
* `const npc *guy` was `nullptr`
* This was in turn caused by `talker::get_character` returning `nullptr`
* The reason for that `nullptr` was that `const talker &u` was of type `talker_character`, created from `get_talker_for( const Creature &me )` in `creature.cpp`
* `talker_character` previously did not override `get_character`, so it used the default impl from `talker` which always returns `nullptr`.

Therefore, this commit updates `talker_character` so that `get_character` returns a valid pointer.

src/npctalk.cpp

@@ -2041,8 +2041,9 @@ void parse_tags( std::string &phrase, const talker &u, const talker &me, const d
             int price = total_price( u, item_type );
             phrase.replace( fa, l, format_money( price ) );
         } else if( tag == "<interval>" ) {
-            const npc *guy = dynamic_cast<const npc *>( &me );
-            phrase.replace( fa, l, guy->get_restock_interval() );
+            const npc *guy = dynamic_cast<const npc *>( me_chr );
+            std::string restock_interval = guy ? guy->get_restock_interval() : _( "a few days" );
+            phrase.replace( fa, l, restock_interval );
         } else if( tag.find( "<u_val:" ) != std::string::npos ) {
             //adding a user variable to the string
             std::string var = tag.substr( tag.find( ':' ) + 1 );

src/talker_character.h

@@ -32,6 +32,20 @@ class talker_character_const: public talker_cloner<talker_character_const>
         }
         ~talker_character_const() override = default;
 
+        // underlying element accessor functions
+        Character *get_character() override {
+            return nullptr;
+        }
+        const Character *get_character() const override {
+            return me_chr_const;
+        }
+        Creature *get_creature() override {
+            return nullptr;
+        }
+        const Creature *get_creature() const override {
+            return me_chr_const;
+        }
+
         // identity and location
         std::string disp_name() const override;
         std::string get_name() const override;