GDScript: Fix crash caused by uninitialized temp stack slots

This adds initialization to every typed temporary stack slot at the
beginning of the function call instead of emitting instructions, since
those might be in a conditional branch and not be called.

modules/gdscript/gdscript_function.h

@@ -496,6 +496,8 @@ private:
 	Vector<GDScriptDataType> argument_types;
 	GDScriptDataType return_type;
 
+	Map<int, Variant::Type> temporary_slots;
+
 #ifdef TOOLS_ENABLED
 	Vector<StringName> arg_names;
 	Vector<Variant> default_arg_values;