patch 9.0.0516: virtual text "above" highlights gap after it

Problem:    Virtual text "above" highlights gap after it.
Solution:   Do not highlight the gap. (closes #11138)
diff --git a/src/drawline.c b/src/drawline.c
index aa75190..0c6b9ab 100644
--- a/src/drawline.c
+++ b/src/drawline.c
@@ -440,7 +440,7 @@
 		*n_extra = n_used + before + after + padding;
 		*n_attr = mb_charlen(*p_extra);
 		if (above)
-		    *n_attr -= padding;
+		    *n_attr -= padding + after;
 		*n_attr_skip = before + padding + col_off;
 	    }
 	}