DO NOT MERGE Update libpng to 1.6.20

BUG:23265085
Change-Id: I85199805636d771f3597b691b63bc0bf46084833
(cherry picked from commit bbe98b40cda082024b669fa508931042eed18f82)

static size_t safecatd(char *buffer, size_t bufsize, size_t pos, double d,
 int precision)
{
 char number[64];
   sprintf(number, "%.*f", precision, d);
 return safecat(buffer, bufsize, pos, number);

 }

static size_t safecatd(char *buffer, size_t bufsize, size_t pos, double d,
 int precision)
{
 char number[64];
   sprintf(number, "%.*f", precision, d);
 return safecat(buffer, bufsize, pos, number);

 }
