[Ada] Prevent crash in Is_Reachable

This patch fixes a bug in Is_Reachable, which causes a crash when checks
are on.

2019-08-12  Bob Duff  <duff@adacore.com>

gcc/ada/

	* libgnat/a-cbmutr.adb (Is_Reachable): Declare Idx to be of the
	base subtype.  Clearly it makes no sense to loop "while Idx >=
	0", if Idx is of a nonnegative subtype.

From-SVN: r274301

gcc/ada/ChangeLog

 2019-08-12  Bob Duff  <duff@adacore.com>
 
+	* libgnat/a-cbmutr.adb (Is_Reachable): Declare Idx to be of the
+	base subtype.  Clearly it makes no sense to loop "while Idx >=
+	0", if Idx is of a nonnegative subtype.
+
+2019-08-12  Bob Duff  <duff@adacore.com>
+
 	* libgnat/a-tifiio.adb (Put_Scaled): Prevent AA from being
 	negative, since Field is range 0 .. something.
 

gcc/ada/libgnat/a-cbmutr.adb

@@ -1767,10 +1767,8 @@ package body Ada.Containers.Bounded_Multiway_Trees is
      (Container : Tree;
       From, To  : Count_Type) return Boolean
    is
-      Idx : Count_Type;
-
+      Idx : Count_Type'Base := From;
    begin
-      Idx := From;
       while Idx >= 0 loop
          if Idx = To then
             return True;