Optimize isBlockAliasedFromMasterCollection

* Fix PHPDoc: $b is a Block instance, not an array
* Fix argument: declaring that $b is passed by reference is useless
  (so let's use $b instead of &$b)
* If $b is not an alias let's return false without filling
   blocksAliasedFromMasterCollection (that's useless)
* $db->execute is useless, since it's already called inside $db->GetCol
Merge pull request #1452 from mlocati/fix-isBlockAliasedFromMasterCollection

Optimize isBlockAliasedFromMasterCollection