libpng: upgrade 1.6.38 -> 1.6.39

Changelog:
==========
*  Changed the error handler of oversized chunks (i.e. larger than
   PNG_USER_CHUNK_MALLOC_MAX) from png_chunk_error to png_benign_error.
*  Fixed a buffer overflow error in contrib/tools/pngfix.
*  Fixed a memory leak (CVE-2019-6129) in contrib/tools/pngcp.
*  Disabled the ARM Neon optimizations by default in the CMake file,
   following the default behavior of the configure script.
*  Allowed configure.ac to work with the trunk version of autoconf.
*  Removed the support for "install" targets from the legacy makefiles;
   removed the obsolete makefile.cegcc.
*  Cleaned up the code and updated the internal documentation.

Signed-off-by: Wang Mingyu <wangmy@fujitsu.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>