appDisplay: Don't have goToPage clamp to negative page index

appDisplay: Don't have goToPage clamp to negative page index

The nPages property can be 0, which is the case in a stripped down CI
image without a single app to be listed, which means we can't clamp to
`nPages - 1`, since that'll give us a negative index and the following
error:

(gnome-shell:266): Gjs-CRITICAL **: 16:48:56.633: JS ERROR: Error: Page -2 does not exist at IconGrid
getItemsAtPage@resource:///org/gnome/shell/ui/iconGrid.js:1383:19
_translatePreviousPageIcons@resource:///org/gnome/shell/ui/appDisplay.js:323:34
_syncPageIndicators@resource:///org/gnome/shell/ui/appDisplay.js:396:14
goToPage@resource:///org/gnome/shell/ui/appDisplay.js:445:14
goToPage@resource:///org/gnome/shell/ui/appDisplay.js:1236:29
goToPage@resource:///org/gnome/shell/ui/appDisplay.js:1582:15
_init/<@resource:///org/gnome/shell/ui/appDisplay.js:499:18
_init/pagesChangedId<@resource:///org/gnome/shell/ui/iconGrid.js:1137:24
_removePage@resource:///org/gnome/shell/ui/iconGrid.js:454:14
_removeItemData@resource:///org/gnome/shell/ui/iconGrid.js:495:18
moveItem@resource:///org/gnome/shell/ui/iconGrid.js:854:14
moveItem@resource:///org/gnome/shell/ui/iconGrid.js:1277:29
_moveItem@resource:///org/gnome/shell/ui/appDisplay.js:1194:20
_redisplay/<@resource:///org/gnome/shell/ui/appDisplay.js:1085:22
_redisplay@resource:///org/gnome/shell/ui/appDisplay.js:1080:17
_redisplay@resource:///org/gnome/shell/ui/appDisplay.js:1370:15
_init/<@resource:///org/gnome/shell/ui/appDisplay.js:633:24
_initializeManager@resource:///org/gnome/shell/misc/parentalControlsManager.js:95:14
async*_init@resource:///org/gnome/shell/misc/parentalControlsManager.js:63:14
ParentalControlsManager@resource:///org/gnome/shell/misc/parentalControlsManager.js:55:4
getDefault@resource:///org/gnome/shell/misc/parentalControlsManager.js:42:22
start@resource:///org/gnome/shell/ui/main.js:184:29
@resource:///org/gnome/shell/ui/init.js:6:17

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1349>