patch 8.2.3012: when 'rightleft' is set the line number is drawn reversed

Problem:    When 'rightleft' is set the line number is sometimes drawn
            reversed.
Solution:   Adjust how space is handled. (Christian Brabandt, closes #8389,
            closes #8391)
diff --git a/src/drawline.c b/src/drawline.c
index 7b6777e..b1210b7 100644
--- a/src/drawline.c
+++ b/src/drawline.c
@@ -1121,8 +1121,9 @@
 			    int		t;
 
 			    // like rl_mirror(), but keep the space at the end
-			    p2 = skiptowhite(extra) - 1;
-			    for (p1 = extra; p1 < p2; ++p1, --p2)
+			    p2 = skipwhite(extra);
+			    p2 = skiptowhite(p2) - 1;
+			    for (p1 = skipwhite(extra); p1 < p2; ++p1, --p2)
 			    {
 				t = *p1;
 				*p1 = *p2;