Use correct order when unreading files from lexer buffer.

Use correct order when unreading files from lexer buffer.

In lexer buffer nested files come before outer files. In lexer file
stack, however, outer files go before nested files (nested are at the
top). We want to break from the unreading cycle early, therefore we
proceed in reverse order of file offsets in buffer and break as soon
as the end offset is less than cursor (current position).