Segfault when opening unseekable file on Cygwin

[mattjala]

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