Fix infloop in lsp-mode

* src/indent.c (line_number_display_width): Make sure the selected
window's buffer is current before using display code on it:
redisplay assumes that the window's buffer is current at all
times.  Reported by Evgeny Kurnevsky via lsp-mode's issue 1621,
emacs-lsp/lsp-mode#1621.

src/indent.c

@@ -1967,9 +1967,13 @@ line_number_display_width (struct window *w, int *width, int *pixel_width)
       struct it it;
       struct text_pos startpos;
       bool saved_restriction = false;
+      struct buffer *old_buf = current_buffer;
       ptrdiff_t count = SPECPDL_INDEX ();
       SET_TEXT_POS_FROM_MARKER (startpos, w->start);
       void *itdata = bidi_shelve_cache ();
+
+      /* Make sure W's buffer is the current one.  */
+      set_buffer_internal_1 (XBUFFER (w->contents));
       /* We want to start from window's start point, but it could be
 	 outside the accessible region, in which case we widen the
 	 buffer temporarily.  It could even be beyond the buffer's end
@@ -1998,6 +2002,7 @@ line_number_display_width (struct window *w, int *width, int *pixel_width)
       *pixel_width = it.lnum_pixel_width;
       if (saved_restriction)
 	unbind_to (count, Qnil);
+      set_buffer_internal_1 (old_buf);
       bidi_unshelve_cache (itdata, 0);
     }
 }