DO NOT MERGE Update libpng to 1.6.20

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

static void rpng2_win_cleanup()
{
 if (bg_image && bg_data) {
        free(bg_data);
        bg_data = NULL;
 }

 if (rpng2_info.image_data) {
        free(rpng2_info.image_data);
        rpng2_info.image_data = NULL;
 }

 if (rpng2_info.row_pointers) {
        free(rpng2_info.row_pointers);
        rpng2_info.row_pointers = NULL;
 }

 if (dib) {
        free(dib);
        dib = NULL;
 }
}

static void rpng2_win_cleanup()
{
 if (bg_image && bg_data) {
        free(bg_data);
        bg_data = NULL;
 }

 if (rpng2_info.image_data) {
        free(rpng2_info.image_data);
        rpng2_info.image_data = NULL;
 }

 if (rpng2_info.row_pointers) {
        free(rpng2_info.row_pointers);
        rpng2_info.row_pointers = NULL;
 }

 if (dib) {
        free(dib);
        dib = NULL;
 }
}
