fix: handle zero-sized and undersized temp buffers in buffered JSON dump

[rgerhards]

Motivation

• Prevent out-of-bounds writes and undefined behavior when callers pass a zero or too-small temporary buffer to the buffered dump API, which could underflow the computed length passed to vsnprintf() and reuse a consumed va_list.

Description

• Make buffer_printf() check the actual available space (available = buffer->size - buffer->filled) and skip calling vsnprintf() when available == 0, forcing the code path that uses a dynamically allocated buffer instead.
• Ensure each formatting attempt initializes and finalizes its own va_list (va_start/va_end) so a consumed va_list is not reused.
• Use available for success comparisons and update buffer->filled (not buffer->size) when writing retry data into the buffer.
• Avoid flushing an empty user-supplied buffer by only calling buffer_flush() when buffer->filled > 0.
• Document that fjson_object_dump_buffered() may be called with temp == NULL when size == 0.
• Add a regression test (tests/test_dump_buffered.{c,test,expected}) and register it in tests/Makefile.am to cover both size == 0 and a very small nonzero size cases.

Testing

• Compiled and ran the new regression test with a normal build and verified output matches tests/test_dump_buffered.expected using gcc and a local config.h (test passed).
• Compiled and ran the same regression test under AddressSanitizer/UndefinedBehaviorSanitizer (-fsanitize=address,undefined) and observed no sanitizer errors and expected output (test passed).
• Ran git diff --check to ensure no whitespace or diff issues (passed).
• autoreconf -fi && ./configure && make check could not be executed in this environment because aclocal is not available (tooling limitation).

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Codex Task

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Summary by cubic

Safely handle zero-sized and undersized temp buffers in the buffered JSON dump to prevent out-of-bounds writes and undefined behavior. Falls back to dynamic allocation when needed and adds a regression test.

• Bug Fixes
  • Skip vsnprintf() when available space is 0; force dynamic buffer path.
  • Start/end a fresh va_list for each formatting attempt.
  • Use available = size - filled for checks; update buffer->filled on retries.
  • Only flush when buffer->filled > 0 to avoid empty-buffer flushes.
  • Document that temp may be NULL when size == 0 in fjson_object_dump_buffered().
  • Add test_dump_buffered covering size == 0 and tiny nonzero buffers; register in tests/Makefile.am and include test_dump_buffered.expected in EXTRA_DIST.

^{Written for commit 2e0085a. Summary will update on new commits.}