softwrap: initialize the 'extrarows' value for the magic line correctly

The make_new_node() function initializes 'extrarows' to a bad value
on purpose, to catch cases like this one where it doesn't get updated
properly.

This fixes https://savannah.gnu.org/bugs/?58890.
Reported-by: Jerry Kindall <kindall@gmail.com>

Bug existed since version 5.0, since the indicator was introduced.

src/utils.c

@@ -431,6 +431,9 @@ void new_magicline(void)
 	openfile->filebot->next = make_new_node(openfile->filebot);
 	openfile->filebot->next->data = copy_of("");
 	openfile->filebot = openfile->filebot->next;
+#ifndef NANO_TINY
+	openfile->filebot->extrarows = 0;
+#endif
 	openfile->totsize++;
 }