ada: Extend precondition of Interfaces.C.String.Value with Length

The existing precondition guarded against exception Dereference_Error,
but not against Constraint_Error.

The RM rule B.3.1(36/3) only mentions Constraint_Error for the Value
function which returns char_array, but the one which returns String
has the same restriction, because it is equivalent to calling the
variant which returns char_array and then converted.

gcc/ada/

	* libgnat/i-cstrin.ads (Value): Extend preconditions; adapt comment for
	the package.

gcc/ada/libgnat/i-cstrin.ads

@@ -36,8 +36,8 @@
 --  Preconditions in this unit are meant for analysis only, not for run-time
 --  checking, so that the expected exceptions are raised. This is enforced by
 --  setting the corresponding assertion policy to Ignore. These preconditions
---  protect from Dereference_Error and Update_Error, but not from
---  Storage_Error.
+--  protect from Constraint_Error, Dereference_Error and Update_Error, but not
+--  from Storage_Error.
 
 pragma Assertion_Policy (Pre => Ignore);
 
@@ -95,7 +95,7 @@ is
      (Item   : chars_ptr;
       Length : size_t) return char_array
    with
-     Pre    => Item /= Null_Ptr,
+     Pre    => Item /= Null_Ptr and then Length /= 0,
      Global => (Input => C_Memory);
 
    function Value (Item : chars_ptr) return String with
@@ -106,7 +106,7 @@ is
      (Item   : chars_ptr;
       Length : size_t) return String
    with
-     Pre    => Item /= Null_Ptr,
+     Pre    => Item /= Null_Ptr and then Length /= 0,
      Global => (Input => C_Memory);
 
    function Strlen (Item : chars_ptr) return size_t with