patch 9.0.0975: virtual text below empty line misplaced when 'number' set

Problem:    Virtual text below an empty line is misplaced when 'number' is
            set.
Solution:   Adjust the computations. (closes #11629)
diff --git a/src/drawline.c b/src/drawline.c
index 8b19662..9347fda 100644
--- a/src/drawline.c
+++ b/src/drawline.c
@@ -663,9 +663,9 @@
 		before -= cells;
 
 	    // Below-align: empty line add one character
-	    if (below && vcol == 0 && col_with_padding == 0
-						      && wp->w_width == before)
-		col_with_padding = 1;
+	    if (below && vcol == 0 && col_with_padding == col_off
+					    && wp->w_width - col_off == before)
+		col_with_padding += 1;
 
 	    if (before < 0
 		    || !(right || below)