This fixes a reproducible indexed-gzip failure when random access resumes at a non-byte-aligned access point.

Without the patch, a gzip-compressed full-disk image exposes its GPT successfully, but filesystem reads fail repeatedly with Z_DATA_ERROR (-3).

With the patch applied to the same nbdkit source commit, the ext4 partition mounts read-only and file reads succeed.

The corresponding GitLab issue is:

nbdkit#32

A detailed unpatched-versus-patched test result is included in the commit message.