Added an additional safeguard (#6241)

Fixes #6229

Thanks to @mrboob

src/map/pc.cpp

@@ -7227,7 +7227,7 @@ int pc_checkbaselevelup(struct map_session_data *sd) {
 void pc_baselevelchanged(struct map_session_data *sd) {
 	uint8 i;
 	for( i = 0; i < EQI_MAX; i++ ) {
-		if( sd->equip_index[i] >= 0 ) {
+		if( sd->equip_index[i] >= 0 && sd->inventory_data[sd->equip_index[i]] ) {
 			if( sd->inventory_data[ sd->equip_index[i] ]->elvmax && sd->status.base_level > (unsigned int)sd->inventory_data[ sd->equip_index[i] ]->elvmax )
 				pc_unequipitem(sd, sd->equip_index[i], 3);
 		}