Fix wrong maybe-uninitialized

[ehughsbaird]

Summary

None

Purpose of change

My compiler was warning me about this, and while the compiler is obviously wrong, it costs nothing to "fix" it.

src/item_factory.cpp: In member function ‘void armor_portion_data::deserialize(const JsonObject&)’:
src/item_factory.cpp:2840:63: error: ‘temp_enum’ may be used uninitialized [-Werror=maybe-uninitialized]
 2840 |         breathability = material_type::breathability_to_rating( temp_enum );
      |                         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~
src/item_factory.cpp:2837:26: note: ‘temp_enum’ was declared here
 2837 |     breathability_rating temp_enum;
      |                          ^~~~~~~~~