[Ada] Add adequate guard before calling First_Rep_Item

[Pierre-Marie de Rodat via Gcc-patches]

New contracts on Ada.Strings.Bounded revealed an unprotected call to
First_Rep_Item on a possibly empty node.

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

gcc/ada/

        * sem_ch13.adb (Build_Predicate_Functions): Add guard.
diff --git a/gcc/ada/sem_ch13.adb b/gcc/ada/sem_ch13.adb
--- a/gcc/ada/sem_ch13.adb
+++ b/gcc/ada/sem_ch13.adb
@@ -10104,7 +10104,10 @@ package body Sem_Ch13 is
          --  If the type is private, check whether full view has inherited
          --  predicates.
 
-         if Is_Private_Type (Typ) and then No (Ritem) then
+         if Is_Private_Type (Typ)
+           and then No (Ritem)
+           and then Present (Full_View (Typ))
+         then
             Ritem := First_Rep_Item (Full_View (Typ));
          end if;