DO NOT MERGE Update libpng to 1.6.20

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

modifier_current_encoding(PNG_CONST png_modifier *pm, color_encoding *ce)
modifier_current_encoding(const png_modifier *pm, color_encoding *ce)
 {
    if (pm->current_encoding != 0)
       *ce = *pm->current_encoding;

 else
      memset(ce, 0, sizeof *ce);

   ce->gamma = pm->current_gamma;

 }

modifier_current_encoding(PNG_CONST png_modifier *pm, color_encoding *ce)
 {
    if (pm->current_encoding != 0)
       *ce = *pm->current_encoding;

 else
      memset(ce, 0, sizeof *ce);

   ce->gamma = pm->current_gamma;

 }
