BUGFIX Type checking in CMS URL params (fixes #7187)

Nasty issue where nested requests to sub-controllers within LeftAndMain->EditForm() would fail because of
LeftAndMain->currentPageID() being taken from the URL, even though it was a field action name.
Example: POST /admin/pages/edit/EditForm/field/MyGridField/item/1/ItemEditForm/field/MyUploadField/upload
The current ID would be detected as "field" in this case.
Bug was previously fixed for AssetAdmin.

admin/code/LeftAndMain.php

@@ -1170,7 +1170,7 @@ function getSilverStripeNavigator() {
 	 * @return int 
 	 */
 	public function currentPageID() {
-		if($this->request->requestVar('ID'))	{
+		if($this->request->requestVar('ID') && is_numeric($this->request->requestVar('ID')))	{
 			return $this->request->requestVar('ID');
 		} elseif (isset($this->urlParams['ID']) && is_numeric($this->urlParams['ID'])) {
 			return $this->urlParams['ID'];