tokener: validate depth and printbuf, guard key strdup

[prownd]

• fjson_tokener_new_ex() rejected no depth check: depth <= 0 called calloc(0, ...) and fjson_tokener_reset() then wrote stack[0], a heap buffer overflow. Reject depth < 1 up front.
• fjson_tokener_new_ex() ignored a printbuf_new() failure, leaving tok->pb == NULL to be dereferenced during parsing. Check it and clean up on failure.
• the object-key strdup() failure was unchecked; a NULL key is later treated as an empty slot and also passed to strcmp(). Bail out with an error instead.

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Summary by cubic

Hardens tokener initialization and object-key parsing against allocation failures. fjson_tokener_new_ex() now rejects depths below 1 and cleans up when printbuf_new() fails, while parsing aborts with an error if duplicating an object key fails instead of continuing with a NULL key.

^{Written for commit 8db922e. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Re-trigger cubic}