Fix crash when confirming signal with "E"

Would happen in certain circumstances when picking a new signal for a
slot that has previously had a signal.

Reported in:
https://mods.factorio.com/mod/cybersyn-combinator/discussion/67515fb722fc1663076c8322

src/changelog.txt

@@ -1,12 +1,15 @@
 ---------------------------------------------------------------------------------------------------
 Version: 2.4.0
-Date: 2024-12-04
+Date: 2024-12-05
   Major Features:
     - Support for logistic sections and groups.
   Features:
     - Show circuit network connection info in GUI.
   Optimizations:
     - Better detection of existing signals/sections when copy-pasting.
+  Bugfixes:
+    - Fix pressing confirm button (default "E") while signal picker dialog is open causing a crash
+      under certain circumstances.
   Gui:
     - Make UI more consistent with base game UI by only allowing left click on some UI elements.
 ---------------------------------------------------------------------------------------------------

src/scripts/gui.lua

@@ -735,6 +735,7 @@ local function confirm_signal_value(player_index, state, clear_selected)
   local slot_button = state.selected_slot_button
   if slot_button and slot_button.valid then slot_button.style = "flib_slot_button_default" end
   local current = state.combinator:get_item_slot(state.selected_section_index, state.selected_slot)
+  if not current.signal or not current.signal.name then return end
   local value = resolve_textfield_number(state.signal_value_items, player_index, current.count or 0)
   if not value then
     state.signal_value_confirm.enabled = false