server: fix client-triggerable double-free in JSON request parsing

[ferdinandobons]

What

Fix a client-triggerable double-free in the HTTP JSON request parsers in ds4_server.c.

The request parsers free a string field and then re-parse into the same field:

free(r->model);
if (!json_string(&p, &r->model)) { /* ... */ goto bad; }

The readers — json_string, json_content, json_raw_value, parse_prompt, parse_anthropic_system, parse_responses_content_array — never write *out on their failure paths. So on a malformed value the field keeps pointing at the just-freed buffer, and the error path frees it again (request_free, a cleanup label bad:/done:/fail:/item_fail:, or tool_call_free) → a double-free fully controlled by the request body.

Impact

r->model is initialized non-NULL by request_init(), so the double-free fires on the first malformed occurrence — no duplicate key needed:

POST /v1/chat/completions   {"model": 5}
POST /v1/messages           {"model": 123}
POST /v1/responses , /v1/completions   likewise

Every other free-then-reparse field is the same class, reachable via a duplicated object key whose second value is malformed, e.g. a content block {"type":"text","type":5} or tool_calls {"arguments":"x","arguments":"\u"}.

Fix

Set the pointer to NULL immediately after the free() at every free-then-reparse site (48 sites). Safe and behaviour-preserving:

• On failure the reader doesn't touch *out, so the field stays NULL and the cleanup free() is a no-op.
• On success the field is overwritten by the parsed value, so valid input is unchanged. No leak is introduced (the old value was already freed before the re-parse).

Testing — AddressSanitizer

A harness that #includes ds4_server.c and calls the real parsers (no reimplementation). Three independent triggers, each hitting a different cleanup path:

trigger	endpoint path
{"model": 5}	parse_chat_request → request_free
[{"function":{"arguments":"x","arguments":"\u"}}]	parse_function_call → tool_call_free
{"prompt":"x","prompt":"\u"}	parse_completion_request cleanup

• Unpatched (main): all three abort with AddressSanitizer: attempting double-free.
• With this change: all three return cleanly (invalid JSON request), no double-free.
• Builds clean with the project flags (-O3 -ffast-math -Wall -Wextra -std=c99); a completeness scan finds no remaining free-then-reparse site without a following = NULL.

No functional change on valid requests.