Code:  image_transform_png_set_tRNS_to_alpha_add(image_transform *this,
    PNG_CONST image_transform **that, png_byte colour_type, png_byte bit_depth)
 {
    UNUSED(bit_depth)
 
 this->next = *that;
 *that = this;

 
    /* We don't know yet whether there will be a tRNS chunk, but we know that
     * this transformation should do nothing if there already is an alpha
    * channel.
     */
   return (colour_type & PNG_COLOR_MASK_ALPHA) == 0;
 }

Vulnerability Type: +Priv 
CWE ID: 
Summary: Unspecified vulnerability in libpng before 1.6.20, as used in Android 4.x before 4.4.4, 5.0.x before 5.0.2, 5.1.x before 5.1.1, and 6.x before 2016-07-01, allows attackers to gain privileges via a crafted application, as demonstrated by obtaining Signature or SignatureOrSystem access, aka internal bug 23265085.
Commit Message: DO NOT MERGE Update libpng to 1.6.20

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