png: update to 1.6.34.

Version 1.6.33beta01 [August 28, 2017]
  Added PNGMINUS_UNUSED macro to contrib/pngminus/p*.c and added missing
    parenthesis in contrib/pngminus/pnm2png.c (bug report by Christian Hesse).
  Fixed off-by-one error in png_do_check_palette_indexes() (Bug report
    by Mick P., Source Forge Issue #269).

Version 1.6.33beta02 [September 3, 2017]
  Initialize png_handler.row_ptr in contrib/oss-fuzz/libpng_read_fuzzer.cc
    to fix shortlived oss-fuzz issue 3234.
  Compute a larger limit on IDAT because some applications write a deflate
    buffer for each row (Bug report by Andrew Church).
  Use current date (DATE) instead of release-date (RDATE) in last
    changed date of contrib/oss-fuzz files.
  Enabled ARM support in CMakeLists.txt (Bernd Kuhls).

Version 1.6.33beta03 [September 14, 2017]
  Fixed incorrect typecast of some arguments to png_malloc() and
    png_calloc() that were png_uint_32 instead of png_alloc_size_t
    (Bug report by "irwir" in Github libpng issue #175).
  Use pnglibconf.h.prebuilt when building for ANDROID with cmake (Github
    issue 162, by rcdailey).

Version 1.6.33rc01 [September 20, 2017]
  Initialize memory allocated by png_inflate to zero, using memset, to
    stop an oss-fuzz "use of uninitialized value" detection in png_set_text_2()
    due to truncated iTXt or zTXt chunk.
  Initialize memory allocated by png_read_buffer to zero, using memset, to
    stop an oss-fuzz "use of uninitialized value" detection in
    png_icc_check_tag_table() due to truncated iCCP chunk.
  Removed a redundant test (suggested by "irwir" in Github issue #180).

Version 1.6.33rc02 [September 23, 2017]
  Added an interlaced version of each file in contrib/pngsuite.
  Relocate new memset() call in pngrutil.c.
  Removed more redundant tests (suggested by "irwir" in Github issue #180).
  Add support for loading images with associated alpha in the Simplified
    API (Samuel Williams).

Version 1.6.33 [September 28, 2017]
  Revert contrib/oss-fuzz/libpng_read_fuzzer.cc to libpng-1.6.32 state.
  Initialize png_handler.row_ptr in contrib/oss-fuzz/libpng_read_fuzzer.cc
  Add end_info structure and png_read_end() to the libpng fuzzer.

Version 1.6.34 [September 29, 2017]
  Removed contrib/pngsuite/i*.png; some of these were incorrect and caused
    test failures.

(wiz)