Log Message:
Updated png to 1.6.23.

Version 1.6.23beta01 [May 29, 2016]
  Stop a potential memory leak in png_set_tRNS() (Bug report by Ted Ying).
  Fixed the progressive reader to handle empty first IDAT chunk properly
    (patch by Timothy Nikkel).  This bug was introduced in libpng-1.6.0 and
    only affected the libpng16 branch.
  Added tests in pngvalid.c to check zero-length IDAT chunks in various
    positions.  Fixed the sequential reader to handle these more robustly
    (John Bowler).

Version 1.6.23rc01 [June 2, 2016]
  Corrected progressive read input buffer in pngvalid.c. The previous version
    the code invariably passed just one byte at a time to libpng.  The intent
    was to pass a random number of bytes in the range 0..511.
  Moved sse2 prototype from pngpriv.h to contrib/intel/intel_sse.patch.
  Added missing ")" in pngerror.c (Matt Sarrett).

Version 1.6.23rc02 [June 4, 2016]
  Fixed undefined behavior in png_push_save_buffer(). Do not call
    memcpy() with a null source, even if count is zero (Leon Scroggins III).

Version 1.6.23 [June 9, 2016]
  Fixed bad link to RFC2083 in png.5 (Nikola Forro).