Corrected Weapon Forging inventory search (#4192)

* Fixes an issue with Weapon Forging not checking for index 0.
Thanks to @Tokeiburu!

src/map/skill.cpp

@@ -19443,10 +19443,10 @@ bool skill_produce_mix(struct map_session_data *sd, uint16 skill_id, unsigned sh
 		make_per += pc_checkskill(sd,skill_id)*500; // Smithing skills bonus: +5/+10/+15
 		make_per += pc_checkskill(sd,BS_WEAPONRESEARCH)*100 +((wlv >= 3)? pc_checkskill(sd,BS_ORIDEOCON)*100:0); // Weaponry Research bonus: +1/+2/+3/+4/+5/+6/+7/+8/+9/+10, Oridecon Research bonus (custom): +1/+2/+3/+4/+5
 		make_per -= (ele?2000:0) + sc*1500 + (wlv>1?wlv*1000:0); // Element Stone: -20%, Star Crumb: -15% each, Weapon level malus: -0/-20/-30
-		if      (pc_search_inventory(sd,ITEMID_EMPERIUM_ANVIL) > 0) make_per+= 1000; // Emperium Anvil: +10
-		else if (pc_search_inventory(sd,ITEMID_GOLDEN_ANVIL) > 0)   make_per+= 500; // Golden Anvil: +5
-		else if (pc_search_inventory(sd,ITEMID_ORIDECON_ANVIL) > 0) make_per+= 300; // Oridecon Anvil: +3
-		else if (pc_search_inventory(sd,ITEMID_ANVIL) > 0)          make_per+= 0; // Anvil: +0?
+		if      (pc_search_inventory(sd,ITEMID_EMPERIUM_ANVIL) > -1) make_per+= 1000; // Emperium Anvil: +10
+		else if (pc_search_inventory(sd,ITEMID_GOLDEN_ANVIL) > -1)   make_per+= 500; // Golden Anvil: +5
+		else if (pc_search_inventory(sd,ITEMID_ORIDECON_ANVIL) > -1) make_per+= 300; // Oridecon Anvil: +3
+		else if (pc_search_inventory(sd,ITEMID_ANVIL) > -1)          make_per+= 0; // Anvil: +0?
 		if (battle_config.wp_rate != 100)
 			make_per = make_per * battle_config.wp_rate / 100;
 	}