patch 8.2.4393: possible number overflow with nested folds

Problem:    Possible number overflow with nested folds.
Solution:   Avoid a negative line number.
diff --git a/src/fold.c b/src/fold.c
index 3f8de1c..33c2699 100644
--- a/src/fold.c
+++ b/src/fold.c
@@ -1604,7 +1604,7 @@
 		    if (amount == MAXLNUM)
 		    {
 			foldMarkAdjustRecurse(&fp->fd_nested,
-				  line1 - fp->fd_top,
+				  0,
 				  line2 - fp->fd_top,
 				  amount,
 				  amount_after + (fp->fd_top - top));
@@ -1614,7 +1614,7 @@
 		    else
 		    {
 			foldMarkAdjustRecurse(&fp->fd_nested,
-				  line1 - fp->fd_top,
+				  0,
 				  line2 - fp->fd_top,
 				  amount,
 				  amount_after - amount);