[Ada] Don't crash on ghost packages when emitting CUDA symbols in ALI files

[Pierre-Marie de Rodat via Gcc-patches]

Before this commit, a GNAT compiled with assertions would crash when
attempting to emit CUDA symbols in ALI files for spark_mode/ghost
packages, whose content is a single null statement.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

        * lib-writ.adb (Output_CUDA_Symbols): Check for null packages.
diff --git a/gcc/ada/lib-writ.adb b/gcc/ada/lib-writ.adb
--- a/gcc/ada/lib-writ.adb
+++ b/gcc/ada/lib-writ.adb
@@ -403,7 +403,9 @@ package body Lib.Writ is
          Kernel_Elm  : Elmt_Id;
          Kernel      : Entity_Id;
       begin
-         if not Enable_CUDA_Expansion then
+         if not Enable_CUDA_Expansion
+           or else Nkind (Unit_Id) = N_Null_Statement
+         then
             return;
          end if;
          Spec_Id := (if Nkind (Unit_Id) = N_Package_Body