fix still *more* backup_lines() breakage


git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@3140 35c25a1d-7b9e-4130-9fde-d3aeb78583b8
diff --git a/src/text.c b/src/text.c
index 83afd0d..6cea181 100644
--- a/src/text.c
+++ b/src/text.c
@@ -1002,7 +1002,7 @@
 	top = openfile->current->prev;
     else
 	top = openfile->current;
-    for (i = par_len; i > 0; i--) {
+    for (i = par_len; i > 0 && top != NULL; i--) {
 	if (top->lineno == fl_lineno_save)
 	    first_line = top;
 	if (top->lineno == edittop_lineno_save)