bug: client is nil

[izouxv]

No description provided.

[CLAassistant]

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[spf13]

Two issues before this can move forward:

1. CLA not signed. Please sign the CLA at https://cla-assistant.io/spf13/afero — this is required for all contributions.

2. Two unrelated fixes bundled together. The sftpfs nil-client fix (not setting File.client in Create/Open) is a clear correctness bug and should be its own PR. The SymlinkIfPossible behavior change (absolute → relative symlinks) is a separate design decision with different implications and needs its own discussion.

Please split into two PRs after signing the CLA. The sftpfs nil fix is high priority.