Fix a regression introduced in "chore: Clean up the return statements"

This fixes commit 27e548af2518ff8d278b45c40d11ad1bdd68eaa0.

The macro `png_check_sig` has been deprecated and remained untested
for decades. And yet, somehow it escaped from all past API cleanups.

Also update the libpng manual.

Reported-by: Matthieu Darbois
3 files changed