factions: don't duplicate factions loaded from templates and from gsav

When reloading factions from master.gsav after they've already been
loaded from the templates by loading NPCs, find the existing factions
instead of making multiple copies.

src/savegame.cpp

@@ -1585,7 +1585,19 @@ void faction_manager::serialize( JsonOut &jsout ) const
 
 void faction_manager::deserialize( JsonIn &jsin )
 {
-    jsin.read( factions );
+    jsin.start_array();
+    while( !jsin.end_array() ) {
+        faction add_fac;
+        jsin.read( add_fac );
+        faction *old_fac = get( add_fac.id );
+        if( old_fac ) {
+            *old_fac = add_fac;
+            // force a revalidation of add_fac
+            get( add_fac.id );
+        } else {
+            factions.emplace_back( add_fac );
+        }
+    }
 }
 
 void Creature_tracker::deserialize( JsonIn &jsin )

src/savegame_json.cpp

@@ -1527,7 +1527,7 @@ void npc::store( JsonOut &json ) const
     json.member( "previous_mission", previous_mission );
     json.member( "faction_api_ver", faction_api_version );
     if( !fac_id.str().empty() ) { // set in constructor
-        json.member( "my_fac", my_fac->id.c_str() );
+        json.member( "my_fac", fac_id.c_str() );
     }
     json.member( "attitude", static_cast<int>( attitude ) );
     json.member( "previous_attitude", static_cast<int>( attitude ) );