Fix some cases of iteration over BufferLines (closes bgo#638314)

__getitem__ should raise an IndexError if an item beyond the last index
is requested. Previously we did not do this, because we expect requests
beyond the last line; specifically requests for (last line + 1). This
patch changes our behaviour to correctly raise an IndexError if the
request is for anything past (last line + 1).

This fixes some cases of infinite iteration, as __iter__ wants to see
that IndexError.