fix: empty underslung gun doesnt prevent going into firing mode UI

Without this fix having an empty underbarrel mod/gun like the grenade
launcher prevents the player from activating the firing ui via 'f'.
This is especially troublesome as most players, including myself,
only know how to change firing modes via this UI. This effectively
prevents players from switching firing mode to the main weapon.

src/avatar_action.cpp

@@ -675,12 +675,13 @@ bool avatar_action::can_fire_weapon( avatar &you, const map &m, const item &weap
 
     std::vector<std::string> messages;
 
-    const gun_mode &mode = weapon.gun_current_mode();
-    bool check_common = ranged::gunmode_checks_common( you, m, messages, mode );
-    bool check_weapon = ranged::gunmode_checks_weapon( you, m, messages, mode );
-    bool can_use_mode = check_common && check_weapon;
-    if( can_use_mode ) {
-        return true;
+    for( const std::pair<const gun_mode_id, gun_mode> &mode_map : weapon.gun_all_modes() ) {
+        const bool check_common = ranged::gunmode_checks_common( you, m, messages, mode_map.second );
+        const bool check_weapon = ranged::gunmode_checks_weapon( you, m, messages, mode_map.second );
+        const bool can_use_mode = check_common && check_weapon;
+        if( can_use_mode ) {
+            return true;
+        }
     }
 
     for( const std::string &message : messages ) {