Initialize _pool array in tinyxml2.h

[AlbertHungGarmin]

Added {} to the declaration of the _pool array in the tinyxml2.h file. This ensures that all elements of the _pool array are initialized to their default values. This change improves the safety and predictability of the code by preventing uninitialized values.

[leethomason]

I'm not sure when that was added to C++ - old compilers certainly don't like it. (I checked godbolt.) It's minor, so it's probably not worth breaking compatibility.