curl_threads: don't do another malloc if the first fails

[bagder]

No description provided.

[Copilot]

Pull request overview

This PR optimizes error handling in the thread creation function by preventing an unnecessary memory allocation when the initial allocation fails. The change ensures that if the first curlx_malloc for the thread handle fails, the second curlx_malloc for the actual call structure is skipped entirely.

Key changes:

• Initialize ac pointer to NULL instead of immediately allocating memory
• Conditionally allocate ac only if the thread handle t was successfully allocated
• Maintains existing error handling logic at the err label

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