fix still *more* backup_lines() breakage

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@3140 35c25a1d-7b9e-4130-9fde-d3aeb78583b8

src/text.c

@@ -1002,7 +1002,7 @@ filestruct *backup_lines(filestruct *first_line, size_t par_len)
 	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)