DO NOT MERGE Update libpng to 1.6.20

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

find_parameters(png_const_charp what, png_charp param, png_charp *list,
 int nparams)
{
 /* Parameters are separated by '\n' or ':' characters, up to nparams are
    * accepted (more is an error) and the number found is returned.
    */
 int i;
 for (i=0; *param && i<nparams; ++i)
 {
 list[i] = param;
 while (*++param) if (*param == '\n' || *param == ':')
 {
 *param++ = 0; /* Terminate last parameter */
 break; /* And start a new one. */
 }
 }

 if (*param)
 {
      fprintf(stderr, "--insert %s: too many parameters (%s)\n", what, param);
      exit(1);
 }

 list[i] = NULL; /* terminates list */
 return i; /* number of parameters filled in */
}

find_parameters(png_const_charp what, png_charp param, png_charp *list,
 int nparams)
{
 /* Parameters are separated by '\n' or ':' characters, up to nparams are
    * accepted (more is an error) and the number found is returned.
    */
 int i;
 for (i=0; *param && i<nparams; ++i)
 {
 list[i] = param;
 while (*++param) if (*param == '\n' || *param == ':')
 {
 *param++ = 0; /* Terminate last parameter */
 break; /* And start a new one. */
 }
 }

 if (*param)
 {
      fprintf(stderr, "--insert %s: too many parameters (%s)\n", what, param);
      exit(1);
 }

 list[i] = NULL; /* terminates list */
 return i; /* number of parameters filled in */
}
