gui: Make lex_gtk_text_buffer_read() count bytes, not characters.
The N parameter to lex_gtk_text_buffer_read() is a count of bytes,
but the implementation treated it as characters.
Also fixes a memory leak, since gtk_text_iter_get_text()'s caller
is responsible for freeing the memory that it returns.
Reported by Alle <ajk-eis@rvhome.de>.
First investigated by John Darrington <john@darrington.wattle.id.au>.
Reviewed by John Darrington <john@darrington.wattle.id.au>.