Segfault when opening unseekable file on Cygwin

**Describe the bug**
`test_unseekable_file()` results in a segfault when run on Cygwin.

**Expected behavior**
Opening an unseekable file should fail, or at least not segfault.

**Platform (please complete the following information)**
- HDF5 develop as of 4ffeb77
- Cygwin version 2.932
- gcc 12.4.0-3
- Ninja 1.12.0-1
- Build shared libs disabled, since these have issues with Cygwin
- parallel/mpi disabled

**Additional context**
See #4797 for discussion