Avoid assertion violations in calculating line-number width

* src/indent.c (line_number_display_width): Don't use the
window's start marker if it belongs to a different buffer.
(Bug#35429)
1 job for master in 52 minutes and 42 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #1554
test-all

00:52:42