MinPlatformPkg/AcpiTables: Initialize variables before used

MadtStructs, NewMadtTable and MaxMadtStructCount are not initialized
before used or at the proper place. So assign values to them at the
beginning and change the logic when freeing MadtStructs and
NewMadtTable.

Cc: Michael Kubacki <michael.a.kubacki@intel.com>
Cc: Chasel Chiu <chasel.chiu@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Shenglei Zhang <shenglei.zhang@intel.com>
Reviewed-by: Michael Kubacki <michael.a.kubacki@intel.com>
Reviewed-by: Chasel Chiu <chasel.chiu@intel.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>