core: fix display of read marker when all buffers line are unread

The bug is when all buffer lines are unread (so "last_read_line" pointer
is NULL), and that the option weechat.look.read_marker_always_show is on.
The read marker was displayed at bottom after switch to the buffer.

Thanks to Nils Görs.

ChangeLog

@@ -11,6 +11,8 @@ http://weechat.org/files/releasenotes/ReleaseNotes-devel.html[release notes]
 
 == Version 0.4.3 (under dev)
 
+* core: fix display of read marker when all buffers line are unread and that
+  option weechat.look.read_marker_always_show is on
 * core: fix memory leak in regex matching when evaluating expression
 * core: add signals "signal_sighup" and "signal_sigwinch" (terminal resized)
 * core: add command /print, add support of more escaped chars in command

src/gui/curses/gui-curses-window.c

@@ -1302,7 +1302,9 @@ gui_window_switch_to_buffer (struct t_gui_window *window,
     }
 
     if (CONFIG_BOOLEAN(config_look_read_marker_always_show)
-        && set_last_read && !window->buffer->lines->last_read_line)
+        && set_last_read
+        && !window->buffer->lines->last_read_line
+        && !window->buffer->lines->first_line_not_read)
     {
         window->buffer->lines->last_read_line = window->buffer->lines->last_line;
     }