warc writer: free hdr on _popul_ehdr overflow in _warc_header

  warc writer: free hdr on _popul_ehdr overflow in _warc_header

  _warc_header() initialises a local archive_string hdr, fills it via
  _popul_ehdr(), and frees it on the success path at the bottom. The
  intermediate error return on r < 0 (the populated header exceeds
  MAX_HDR_SIZE=512, reachable with a long enough pathname carried by
  WARC-Target-URI) returns ARCHIVE_WARN without freeing hdr.

  By the time _popul_ehdr returns -1 hdr already holds the WARC version
  line, WARC-Type, WARC-Target-URI (long path), WARC-Date, Last-Modified,
  WARC-Record-ID, and Content-Length headers --- ~1 KB+ per entry in
  practice.

  Reproduces with the bundled bsdtar against a regular file with a
  ~400+ char pathname:
      ASAN_OPTIONS=detect_leaks=1 bsdtar --format=warc -cf out.warc <long_path>
  => LeakSanitizer: ~1 KB direct leak; stack pierces _popul_ehdr ->
     _warc_header (archive_write_set_format_warc.c:245) ->
     _archive_write_header -> ... -> main.

  Code unchanged since the file's introduction in 2014; no existing test
  exercises the long-pathname path. The warcinfo branch above (line ~217)
  handles this correctly already (frees hdr unconditionally outside the
  "if (r >= 0)" block); only the WT_RSRC branch was missed.

  Identified by Neurolog, a code-analysis tool the reporter is developing
  that combines Souffle Datalog with LLM-assisted fact extraction. The
  reproducer was hand-validated under LeakSanitizer against current master.