vis: fix newline insertion at end of file

With enabled auto indentation and tab expansion in an empty buffer,
the following would insert one newline too many:

 i<Tab><Enter>

1 files changed, 1 insertions, 1 deletions

diff --git a/vis.c b/vis.c
index 4a38903..bc488f6 100644
--- a/vis.c
+++ b/vis.c
@@ -1596,7 +1596,7 @@ size_t vis_text_insert_nl(Vis *vis, Text *txt, size_t pos) {
 	text_insert(txt, pos, "\n", 1);
 	if (eof) {
 		if (nl2)
-			text_insert(txt, pos, "\n", 1);
+			text_insert(txt, text_size(txt), "\n", 1);
 		else
 			pos--; /* place cursor before, not after nl */
 	}