SCI: Fix GCC warning

[orgads]

Use default initialization for dummy.

In file included from engines/sci/engine/savegame.cpp:26:
In member function 'void Common::Serializer::syncAsUint16LE(T&, Version, Version) [with T = short unsigned int]',
    inlined from 'void Common::Serializer::syncAsUint16LE(T&, Version, Version) [with T = short unsigned int]' at common/serializer.h:120:2,
    inlined from 'void Sci::syncWithSerializer(Common::Serializer&, reg_t&)' at engines/sci/engine/savegame.cpp:77:18,
    inlined from 'void Sci::syncWithSerializer(Common::Serializer&, Node&)' at engines/sci/engine/savegame.cpp:96:20,
    inlined from 'void Sci::SegmentObjTableEntrySyncer<T>::operator()(Common::Serializer&, typename T::Entry&, int) const [with T = Sci::NodeTable]' at engines/sci/engine/savegame.cpp:136:23,
    inlined from 'void Sci::ArraySyncer<T, Syncer>::operator()(Common::Serializer&, Common::Array<T>&) const [with T = Sci::SegmentObjTable<Sci::Node>::Entry; Syncer = Sci::SegmentObjTableEntrySyncer<Sci::NodeTable>]' at engines/sci/engine/savegame.cpp:168:8,
    inlined from 'void Sci::syncArray(Common::Serializer&, Common::Array<T>&) [with T = SegmentObjTable<Node>::Entry; Syncer = SegmentObjTableEntrySyncer<NodeTable>]' at engines/sci/engine/savegame.cpp:183:6,
    inlined from 'void Sci::sync_Table(Common::Serializer&, T&) [with T = NodeTable]' at engines/sci/engine/savegame.cpp:499:62,
    inlined from 'virtual void Sci::NodeTable::saveLoadWithSerializer(Common::Serializer&)' at engines/sci/engine/savegame.cpp:507:23:
common/serializer.h:49:30: warning: 'dummy' may be used uninitialized [-Wmaybe-uninitialized]
   49 |                         TYPE tmp = val; \
      |                              ^~~
common/serializer.h:120:9: note: in expansion of macro 'SYNC_AS'
  120 |         SYNC_AS(Uint16LE, uint16, 2)
      |         ^~~~~~~

[orgads]

The first commit is not needed for the fix, but I still prefer that you review it.

[bluegr]

Thanks. Has this been tested with different SCI games, using a debug build?

[orgads]

I only have SQ5, and not sure how to reach this condition. Tested on release build without NDEBUG :)

[bluegr]

I've tested this with different SCI games. To test, you can always use the SCI demos, found in our demos page.

Thanks! Merging