cfFilterPWGToPDF continues after page preparation fails

Summary

A PWG Raster page with an unsupported color space makes pwgtopdf log Color space not supported and then crash through a null callback.

Reproduction

Build current upstream with ASan/UBSan. Start from a parseable one-page PWG Raster stream (2SaR / PwgRaster sync), retain a complete page header and minimal payload, then set cupsColorSpace to a value not supported by prepare_pdf_page().

ASAN_OPTIONS=detect_leaks=0 PPD=candidate.ppd CONTENT_TYPE=application/vnd.cups-pwg 
  pwgtopdf 1 fuzzer fuzzer 1 'Resolution=300dpi PageSize=Letter' document.pwg >/dev/null

Result

Reproduced through the real pwgtopdf CLI with an ASan/UBSan build from libcupsfilters HEAD d63939da08eaef622a2443b7c9638d22c4d9831d and libppd HEAD 522af8dd135f4dde66b1aac8b9d067808bbe122d on 2026-07-19. The filter logs cfFilterPWGToPDF: Color space not supported, then ASan reports a SEGV in convert_raster() at cupsfilters/pwgtopdf.c:1637, calling doc->bit_function.

Cause and expected behavior

prepare_pdf_page() returns an error without initializing conversion callbacks. add_pdf_page() ignores that result at cupsfilters/pwgtopdf.c:1491, and convert_raster() later calls doc->bit_function. Propagate the preparation failure and abort the job cleanly.

Start in cupsfilters/pwgtopdf.c at add_pdf_page() around line 1491, then follow prepare_pdf_page() and convert_raster() around line 1637. Re-run the supplied ASan/UBSan pwgtopdf command with an unsupported cupsColorSpace; done means the preparation error is propagated and the job aborts cleanly without calling the null conversion callback.