Changed type of yy_n_chars to int; gh#53, sf#160.

Changed type of yy_n_chars to int; gh#53, sf#160.

The variable yy_n_chars had been of type yy_size_t which is incorrect
given its use in read(). While it might be adviseable to look at
defining a yy_ssize_t, there might be some issues doing this and so, for
now, at least, we'll punt back to int.