fix(marks): read from the correct variable in conceal_lines mark collection (#39991)

Problem:  Not using current mark for namespace check during iteration for conceal_line marks.
Solution: Use the appropriate mark variable for namespace check.
(cherry picked from commit 70792aa6e4)

src/nvim/decoration.c

@@ -894,7 +894,7 @@ bool decor_conceal_line(win_T *wp, int row, bool check_cursor)
     if (mark.pos.row > row) {
       break;
     }
-    if (mt_conceal_lines(mark) && ns_in_win(pair.start.ns, wp)) {
+    if (mt_conceal_lines(mark) && ns_in_win(mark.ns, wp)) {
       return true;
     }
     marktree_itr_next_filter(wp->w_buffer->b_marktree, itr, row + 1, 0, conceal_filter);