Fix uninitialized variables.

[walterj-adsk]

clang-tidy found an issue with the cereal library during static
analysis. It indicated that data could end up uninitialized. The fix
involves setting reasonable default values before values are read by
the cereal library.

The contribution is being made by Autodesk Inc. under the terms of the
BSD license.

[crazyhappygame]

Hi could you accept this pull request?

And also initialize with {} other size_types.
There is random crash on mac because of uninitialized size
include/cereal/types/vector.hpp:78: size_type size;

$ git grep -n "^\s*size_type"
include/cereal/types/concepts/pair_associative_container.hpp:52:    size_type size;
include/cereal/types/deque.hpp:52:    size_type size;
include/cereal/types/forward_list.hpp:45:    size_type const size = std::distance( forward_list.begin(), forward_list.end() );
include/cereal/types/forward_list.hpp:58:    size_type size;
include/cereal/types/list.hpp:52:    size_type size;
include/cereal/types/set.hpp:54:      size_type size;
include/cereal/types/string.hpp:53:    size_type size;
include/cereal/types/unordered_set.hpp:54:      size_type size;
include/cereal/types/valarray.hpp:56:    size_type valarraySize;
include/cereal/types/valarray.hpp:80:    size_type valarraySize;
include/cereal/types/vector.hpp:54:    size_type vectorSize;
include/cereal/types/vector.hpp:78:    size_type size;
include/cereal/types/vector.hpp:99:    size_type size;