GUI: Fix check for no usable weapons.

Fixes #3424.

(cherry-picked from commit f293c7c)

src/mouse_events.cpp

@@ -988,7 +988,7 @@ int mouse_handler::show_attack_dialog(const map_location& attacker_loc, const ma
 	std::vector<battle_context> bc_vector;
 	const int best = fill_weapon_choices(bc_vector, attacker, defender);
 
-	if((*attacker).attacks().empty()) {
+	if(bc_vector.empty()) {
 		gui2::show_transient_message("No Attacks", _("This unit has no usable weapons."));
 
 		return -1;