patch 9.1.0815: "above" virtual text causes wrong 'colorcolumn' position

Problem:  "above" virtual text causes wrong 'colorcolumn' position.
          (@matrdr)
Solution: Use the number of cells instead of bytes for vcol_off_tp.
          (zeertzjq)

fixes: #15946
closes: #15948

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/drawline.c b/src/drawline.c
index fd5d56b..b49e653 100644
--- a/src/drawline.c
+++ b/src/drawline.c
@@ -2263,7 +2263,7 @@
 				}
 
 				if (above)
-				    wlv.vcol_off_tp = wlv.n_extra;
+				    wlv.vcol_off_tp = vim_strsize(wlv.p_extra);
 
 				if (lcs_eol_one < 0
 					&& wp->w_p_wrap