[Ada] Issue error on illegal ownership in SPARK

Check for declaration of global variables prior to use in the ownership
checking for SPARK.

There is no impact on compilation.

2019-07-09  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* sem_spark.adb (Get_Perm_Or_Tree): Issue an error when
	encountering unknown global variable.

From-SVN: r273267

gcc/ada/ChangeLog

 2019-07-09  Yannick Moy  <moy@adacore.com>
 
+	* sem_spark.adb (Get_Perm_Or_Tree): Issue an error when
+	encountering unknown global variable.
+
+2019-07-09  Yannick Moy  <moy@adacore.com>
+
 	* sem_spark.adb (Check_Expression): Change signature to take an
 	Extended_Checking_Mode, for handling read permission checking of
 	sub-expressions in an assignment.

gcc/ada/sem_spark.adb

@@ -3270,7 +3270,16 @@ package body Sem_SPARK is
                C : constant Perm_Tree_Access :=
                  Get (Current_Perm_Env, Unique_Entity (Entity (N)));
             begin
-               pragma Assert (C /= null);
+               --  Except during elaboration, the root object should have been
+               --  declared and entered into the current permission
+               --  environment.
+
+               if not Inside_Elaboration
+                 and then C = null
+               then
+                  Illegal_Global_Usage (N);
+               end if;
+
                return (R => Unfolded, Tree_Access => C);
             end;