Don't move cursor forward when it points after last character.

Author: godunko

source/text/implementation/vss-implementation-text_handlers-utf8-dynamic.adb

@@ -576,7 +576,9 @@ package body VSS.Implementation.Text_Handlers.UTF8.Dynamic is
          return;
       end if;
 
-      Unchecked_Forward (Self.Pointer.Storage, After);
+      if To.Index <= Self.Length then
+         Unchecked_Forward (Self.Pointer.Storage, After);
+      end if;
 
       Size   := After.UTF8_Offset - From.UTF8_Offset;
       Length := After.Index - From.Index;

source/text/implementation/vss-implementation-text_handlers-utf8-static.adb

@@ -515,7 +515,9 @@ package body VSS.Implementation.Text_Handlers.UTF8.Static is
          return;
       end if;
 
-      Unchecked_Forward (Self.Storage, After);
+      if To.Index <= Self.Length then
+         Unchecked_Forward (Self.Storage, After);
+      end if;
 
       Size   := After.UTF8_Offset - From.UTF8_Offset;
       Length := After.Index - From.Index;