Overview:
When using Page-up to scroll to the top of a list such as the keyboard shortcuts list in gnome-control-center or to the top of an applications list in the elementary.appcenter(Pop!_Shop), the app crashes.

Steps to Reproduce:
"1. go to Settings->Device[s]->Keyboard
2. click on any entry then press Escape -- this is to get focus for navigating the list with up/down arrows and page up/down
3. press Page Up until reach the top; when it reaches the top entry the Settings window crashes every time"

Actual Result:
Application crashed

Hardware/Platforms:

This issue was found in Pop!_OS 17.10 and confirmed in the upstream Ubuntu 17.10 as well. Initially reported here: https://github.com/pop-os/pop/issues/89

GTK version: 3.22.25

Additional Information:

Output of journald is similar for both the gnome-control-center and elementary.appcenter instances of the respective app crashing:

`Dec 12 20:15:04 [redacted] gnome-control-center.desktop[4218]: **
Dec 12 20:15:04 [redacted] gnome-control-center.desktop[4218]: Gtk:ERROR:/build/gtk+3.0-wTY3BU/gtk+3.0-3.22.25/./gtk/gtklistbox.c:3094:gtk_list_box_move_cursor: assertion failed: (count > 0)`


The assertion that is terminating the application is on line 2838 of gtklistbox.c.

"/* A NULL row should only happen when the list box didn't
  * have enough rows to fill its height and the user made
  * a page movement down, so the count must be positive */"
Seems like there is another corner case to be considered.

Side-note: This bug seems to be a similar error in logic similar to the page-down crashing issue: #770703.