Conf: ensure that the default path is NUL-terminated.

[bavshin-f5]

ngx_merge_path_value takes care of that when it reallocates a resolved relative path, but the reallocation is skipped if we pass an absolute path with NGX_ACME_STATE_PREFIX.

As a result, we get a lovely

drwx------ 2 root  root 4096 Oct  6 21:38  acme_dogtag
drwx------ 2 nginx root 4096 Oct  6 21:38 'acme_dogtag'$'\034'
drwx------ 2 root  root 4096 Oct  6 21:38  acme_stepca
drwx------ 2 nginx root 4096 Oct  6 21:38  acme_stepcaU
drwx------ 2 nginx root 4096 Oct  6 21:38  acme_vault

Note for future: we need to enforce such requirements in a type system, e.g. by requiring CStr for path operations in ngx-rust.

[Copilot]

Pull Request Overview

This PR fixes a critical bug in the default state path generation where paths were not being NUL-terminated when using absolute paths with NGX_ACME_STATE_PREFIX. This caused filesystem corruption with garbled directory names containing unprintable characters.

Key changes:

• Increased memory reservation to account for NUL terminator
• Added explicit NUL byte to path buffer
• Adjusted returned length to exclude NUL terminator from string length

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[xeioex]

Fix ngx_merge_path_value to ngx_conf_merge_path_value in Conf: ensure that the default path is NUL-terminated. commit log.

[xeioex]

Otherwise looks good.