vi: fix reading of file after last line

vi: fix reading of file after last line

If the :r command is used to read a file after the last line of the
buffer the last line of the buffer and the first line of the file
are joined.  An extra blank line appears at the end of the buffer.

   file 1
   file 1
   file 1file 2
   file 2
   file 2

	~
	~

The insertion point is normally at the start of the line following the
specified line.  When the specified line is the last one the next_line
function baulks at moving to the non-existent following line.

Signed-off-by: Ron Yorston <rmy@tigress.co.uk>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>