Update libpng to 1.6.39
This PR updates libpng to latest 1.6.39 from previous 1.6.38.

See below for some issues fixed in 1.6.39:
https://github.com/glennrp/libpng/blob/libpng16/CHANGES#L6112-L6122

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>