vi: fix vda's thinko

diff --git a/editors/vi.c b/editors/vi.c
index 1b335d9..81baa89 100644
--- a/editors/vi.c
+++ b/editors/vi.c
@@ -1894,7 +1894,7 @@
 		p           = new_text + (p           - text);
 		text = new_text;
 	}
-	memmove(p + size, p, end - p);
+	memmove(p + size, p, end - size - p);
 	memset(p, ' ', size);	// clear new hole
 	file_modified++;
 	return p;