(Bug report from the OSS-fuzz project).

when returning with an error.

the unknown-sAPI test is failing.

and png_set_eXIf_1(), respectively, to avoid breaking API compatibility
with libpng-1.6.31.

  in pngwrite.c, and made various other fixes to png_write_eXIf().
Eliminated png_ptr->num_exif member from pngstruct.h and added num_exif
  to arguments for png_get_eXIf() and png_set_eXIf().

are allowed. Avoid leaking the input buffer "eXIf_buf".

(Bug report by John Smith)

tags) to pngtest.png.