[Ada] Avoid crash when traversing units with -gnatd.WW debug switch

[Pierre-Marie de Rodat]

The debug switch -gnatd.WW enables extra info when traversing library units
with Walk_Library_Items, which is used in the CodePeer and GNATprove. This
routine was crashing when trying to print info about a unit with configuration
pragmas (typically an .adc file). Now fixed.

No test, as the crash only happens when a GNATprove backend is manually called
with -gnatd.WW switch. Frontend is not affected.

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

2018-07-16  Piotr Trojanek  <troja...@adacore.com>

gcc/ada/

        * sem.adb (Walk_Library_Items): Skip units with configuration pragmas
        when printing debug info.
--- gcc/ada/sem.adb
+++ gcc/ada/sem.adb
@@ -2242,8 +2242,14 @@ package body Sem is
 
             for Unit_Num in Done'Range loop
                if not Done (Unit_Num) then
-                  Write_Unit_Info
-                    (Unit_Num, Unit (Cunit (Unit_Num)), Withs => True);
+
+                  --  Units with configuration pragmas (.ads files) have empty
+                  --  compilation-unit nodes; skip printing info about them.
+
+                  if Present (Cunit (Unit_Num)) then
+                     Write_Unit_Info
+                       (Unit_Num, Unit (Cunit (Unit_Num)), Withs => True);
+                  end if;
                end if;
             end loop;