Log Message:
Update to 1.6.20:

Version 1.6.20beta01 [November 20, 2015]
  Avoid potential pointer overflow/underflow in png_handle_sPLT() and
    png_handle_pCAL() (Bug report by John Regehr).

Version 1.6.20beta02 [November 23, 2015]
  Fixed incorrect implementation of png_set_PLTE() that uses png_ptr
    not info_ptr, that left png_set_PLTE() open to the CVE-2015-8126
    vulnerability.

Version 1.6.20beta03 [November 24, 2015]
  Backported tests from libpng-1.7.0beta69.

Version 1.6.20rc01 [November 26, 2015]
  Fixed an error in handling of bad zlib CMINFO field in pngfix, found by
    American Fuzzy Lop, reported by Brian Carpenter.  inflate() doesn't
    immediately fault a bad CMINFO field; instead a 'too far back' error
    happens later (at least some times).  pngfix failed to limit CMINFO to
    the allowed values but then assumed that window_bits was in range,
    triggering an assert. The bug is mostly harmless; the PNG file cannot
    be fixed.

Version 1.6.20rc02 [November 29, 2015]
  In libpng 1.6 zlib initialization was changed to use the window size
    in the zlib stream, not a fixed value. This causes some invalid images,
    where CINFO is too large, to display 'correctly' if the rest of the
    data is valid.  This provides a workaround for zlib versions where the
    error arises (ones that support the API change to use the window size
    in the stream).

Version 1.6.20 [December 3, 2015]
  No changes.