libpng: security bump to version 1.6.19

Fixes:
png_set_PLTE/png_get_PLTE functions failed to check for
an out-of-range palette when reading or writing PNG files with a bit_depth
less than 8.

CVE not yet assigned.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>