vis: use strncpy to copy into fixed sized buffer

In practice this was never an issue also it is guaranteed that the
terminating zero byte is already there.

Fixes coverity issue 157023.

vis.c

@@ -425,7 +425,7 @@ static void window_draw_eof(Win *win) {
 		return;
 	CellStyle style = win->ui->style_get(win->ui, UI_STYLE_EOF);
 	for (Line *l = view_lines_last(view)->next; l; l = l->next) {
-		strcpy(l->cells[0].data, view_symbol_eof_get(view));
+		strncpy(l->cells[0].data, view_symbol_eof_get(view), sizeof(l->cells[0].data)-1);
 		l->cells[0].style = style;
 	}
 }