mktables: Fix #23520

The code was attempting to get a slice of an entire array excluding the
0th element.  But it was using the wrong incantation.

The array is the path to a file with each element being one directory.

And it just happened to work because of the current layout of the
directories.  This commit changes so it would continue to work if the
directory structure were to be altered.

Author: khwilliamson

lib/unicore/mktables

@@ -16087,7 +16087,10 @@ sub register_file_for_name($table, $directory_ref, $file) {
         # No swash means don't do the rest of this.
         next if $table->isa('Map_Table') && $table->fate != $ORDINARY;
 
-        my $sub_filename = join('/', $directory_ref->[1, -1], $file);
+        # Get the path to $file stripping off any top level directory
+        my $sub_filename = join('/',
+                                ($directory_ref->@[1, $#$directory_ref] // ""),
+                                $file);
 
         my $property = $table->property;
         my $property_name = ($property == $perl)