Ada: fix ICE with empty separate

[Arnaud Charlet]

Tested on x86-linux

Fixes an ICE on a procedure with an empty separate:

package T is
  procedure Dummy;
end T;
package body T is
   package Load is
   end Load;

   package body Load is separate;

   procedure Dummy is
   begin
     null;
   end;
end T;

$ touch t-load.adb
$ gcc -c -gnatc -gnatQ t.adb

2004-10-26  Ed Schonberg  <schonberg@gnat.com>

	* sem_ch10.adb (Optional_Subunit): If file of expected subunit is
	empty, post message on stub.

-------------- next part --------------
Index: sem_ch10.adb
===================================================================
RCS file: /cvs/gcc/gcc/gcc/ada/sem_ch10.adb,v
retrieving revision 1.30
diff -u -p -r1.30 sem_ch10.adb
--- sem_ch10.adb	26 Jul 2004 10:42:01 -0000	1.30
+++ sem_ch10.adb	27 Oct 2004 09:33:36 -0000
@@ -980,7 +980,14 @@ package body Sem_Ch10 is
          then
             Comp_Unit := Cunit (Unum);
 
-            if Nkind (Unit (Comp_Unit)) /= N_Subunit then
+            --  If the file was empty or seriously mangled, the unit
+            --  itself may be missing.
+
+            if No (Unit (Comp_Unit)) then
+               Error_Msg_N
+                 ("subunit does not contain expected proper body", N);
+
+            elsif Nkind (Unit (Comp_Unit)) /= N_Subunit then
                Error_Msg_N
                  ("expected SEPARATE subunit, found child unit",
                   Cunit_Entity (Unum));