DO NOT MERGE Update libpng to 1.6.20

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

formatof(const char *arg)
{
 char *ep;
 unsigned long format = strtoul(arg, &ep, 0);

 if (ep > arg && *ep == 0 && format < FORMAT_COUNT)
 return (png_uint_32)format;

 else for (format=0; format < FORMAT_COUNT; ++format)
 {
 if (strcmp(format_names[format], arg) == 0)
 return (png_uint_32)format;
 }

   fprintf(stderr, "pngstest: format name '%s' invalid\n", arg);
 return FORMAT_COUNT;
}

formatof(const char *arg)
{
 char *ep;
 unsigned long format = strtoul(arg, &ep, 0);

 if (ep > arg && *ep == 0 && format < FORMAT_COUNT)
 return (png_uint_32)format;

 else for (format=0; format < FORMAT_COUNT; ++format)
 {
 if (strcmp(format_names[format], arg) == 0)
 return (png_uint_32)format;
 }

   fprintf(stderr, "pngstest: format name '%s' invalid\n", arg);
 return FORMAT_COUNT;
}
