Validate cmstarthexsortkey/cmendhexsortkey to be valid hex

Avoids: Warning: pack(): Type H: illegal hex digit N in
\includes\api\ApiQueryCategoryMembers.php on line 146

Bug: 40809
Change-Id: I1dd732ccec8e6991d0ceac443226f5c7e59fd853
Merge "Validate cmstarthexsortkey/cmendhexsortkey to be valid hex"