fix bad node identification for certain SEXP errors

Set the bad node to the ship node, so that the FREDder can see which ship is receiving the invalid order.  Without this change, the bad node would point to the last value parsed, which is typically the goal priority.

Fixes a misleading error dialog reported in Discord.

code/parse/sexp.cpp

@@ -3176,12 +3176,16 @@ int check_sexp_syntax(int node, int return_type, int recursive, int *bad_node, s
 					z = get_operator_const(z);
 					if (ship_num >= 0) {
 						if (!query_sexp_ai_goal_valid(z, ship_num)){
+							if (bad_node)
+								*bad_node = ship_node;
 							return SEXP_CHECK_ORDER_NOT_ALLOWED;
 						}
 
 					} else {
 						for (i=0; i<Wings[wing_num].wave_count; i++){
 							if (!query_sexp_ai_goal_valid(z, Wings[wing_num].ship_index[i])){
+								if (bad_node)
+									*bad_node = ship_node;
 								return SEXP_CHECK_ORDER_NOT_ALLOWED;
 							}
 						}
@@ -3190,6 +3194,8 @@ int check_sexp_syntax(int node, int return_type, int recursive, int *bad_node, s
 					if ((z == OP_AI_DOCK) && (Sexp_nodes[node].rest >= 0)) {
 						ship2 = ship_name_lookup(CTEXT(Sexp_nodes[node].rest), 1);	// Goober5000 - include players
 						if ((ship_num < 0) || !ship_docking_valid(ship_num, ship2)){
+							if (bad_node)
+								*bad_node = ship_node;
 							return SEXP_CHECK_DOCKING_NOT_ALLOWED;
 						}
 					}