Fix compiler warnings in _zoneinfo.c (GH-20342)

```
D:\a\cpython\cpython\Modules\_zoneinfo.c(903,52): warning C4267: '=': conversion from 'size_t' to 'unsigned int', possible loss of data [D:\a\cpython\cpython\PCbuild\_zoneinfo.vcxproj]
D:\a\cpython\cpython\Modules\_zoneinfo.c(904,44): warning C4267: '=': conversion from 'size_t' to 'unsigned int', possible loss of data [D:\a\cpython\cpython\PCbuild\_zoneinfo.vcxproj]
D:\a\cpython\cpython\Modules\_zoneinfo.c(1772,31): warning C4244: '=': conversion from 'ssize_t' to 'uint8_t', possible loss of data [D:\a\cpython\cpython\PCbuild\_zoneinfo.vcxproj]
```