libssh2: fix error handling on quote errors

[bagder]

Previously it lacked the actual return.

Reported-by: m777m0 on hackerone

Follow-up to 578706a

[bagder]

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🔒 Aisle Security Analysis

✅ We scanned this PR and did not find any security vulnerabilities.

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Analyzed PR: #20883 at commit 74a4702

_{Last updated on: 2026-03-11T07:41:30Z}

[augmentcode]

🤖 Augment PR Summary

Summary: Fixes SFTP quote-command parsing in the libssh2 backend to correctly propagate errors when unexpected trailing data is present.

Changes: Adds a dedicated quote_error() helper and updates all affected branches to immediately return CURLE_QUOTE_ERROR after freeing quote paths.

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[Copilot]

Pull request overview

This PR fixes SFTP quote-command error handling in the SSH backends by ensuring the parser actually returns an error when it detects trailing/suspicious data after a command’s arguments, rather than continuing execution after freeing quote-path buffers.

Changes:

• libssh2 backend: make “suspicious data after the command line” paths return CURLE_QUOTE_ERROR immediately (instead of just calling the helper and continuing).
• Rename the quote-error helper to quote_error() in both libssh2 and libssh backends, aligning the helper name with the updated control flow.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lib/vssh/libssh2.c	Ensures quote-parse “trailing data” errors immediately return CURLE_QUOTE_ERROR and renames the helper to quote_error().
lib/vssh/libssh.c	Renames the helper to quote_error() (state-machine path already returned properly).

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