alliance_roles_processing.php: fix PHP warning
alliance_roles_processing.php: fix PHP warning

Checkbox inputs are only posted if they are checked, so we can simply
check for their existence in `$_REQUEST`. This avoids an undefined
index PHP warning.

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