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

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.

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

gcc/ada/

	* sem.adb (Walk_Library_Items): Skip units with configuration pragmas
	when printing debug info.

From-SVN: r262720

gcc/ada/ChangeLog

 2018-07-16  Piotr Trojanek  <trojanek@adacore.com>
 
+	* sem.adb (Walk_Library_Items): Skip units with configuration pragmas
+	when printing debug info.
+
+2018-07-16  Piotr Trojanek  <trojanek@adacore.com>
+
 	* sem.adb (Walk_Library_Items): Reuse local constant.
 	(Is_Subunit_Of_Main): Turn condition to positive and flip the
 	IF-THEN-ELSE branches; avoid potentially ineffective assignment to the

gcc/ada/sem.adb

@@ -2242,9 +2242,15 @@ package body Sem is
 
             for Unit_Num in Done'Range loop
                if not Done (Unit_Num) then
+
+                  --  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;
 
             Outdent;