ada: Fix libgpr2 build failure with compiler built with assertions

The problem is that the Entity field is accessed for a node without one.

gcc/ada/ChangeLog:

	* sem_ch10.adb (Install_Siblings.In_Context): Add missing guard.

gcc/ada/sem_ch10.adb

diff --git a/gcc/ada/sem_ch10.adb b/gcc/ada/sem_ch10.adb
index de5a8c8..e3d9925 100644
--- a/gcc/ada/sem_ch10.adb
+++ b/gcc/ada/sem_ch10.adb
@@ -4932,6 +4932,8 @@
                            if Entity (Name (Clause)) = Id
                              or else
                                (Nkind (Name (Clause)) = N_Expanded_Name
+                                 and then
+                                   Is_Entity_Name (Prefix (Name (Clause)))
                                  and then Entity (Prefix (Name (Clause))) = Id)
                            then
                               return True;