[Ada] Assert failure on invalid Variable_Indexing

gcc/ada/

	* sem_ch4.adb (Try_Container_Indexing): Add ??? comment. Protect
	against malformed tree in case of errors.

gcc/ada/sem_ch4.adb

@@ -286,6 +286,7 @@ package body Sem_Ch4 is
       Prefix : Node_Id;
       Exprs  : List_Id) return Boolean;
    --  AI05-0139: Generalized indexing to support iterators over containers
+   --  ??? Need to provide a more detailed spec of what this function does
 
    function Try_Indexed_Call
      (N          : Node_Id;
@@ -8469,6 +8470,12 @@ package body Sem_Ch4 is
       if not Is_Overloaded (Func_Name) then
          Func := Entity (Func_Name);
 
+         --  Can happen in case of e.g. cascaded errors
+
+         if No (Func) then
+            return False;
+         end if;
+
          Indexing :=
            Make_Function_Call (Loc,
              Name                   => New_Occurrence_Of (Func, Loc),