alliance_stat_processing.php: fix PHP warning
alliance_stat_processing.php: fix PHP warning

If a player has permission to change some alliance info, but *not*
`change_mod` permission, then this will result in the following
warning:

Undefined variable: url in /smr/engine/Default/alliance_stat_processing.php on line 28

Fix this by first checking that `isset($url)` before using it.

smrealms/smr,hemberger/smr,hemberger/smr,smrealms/smr,smrealms/smr,hemberger/smr,hemberger/smr