[libpng16] Fix the calculation of row_factor in png_check_chunk_length

(Bug report by Thuan Pham, SourceForge issue #278)
png_get_uint_16)(png_const_bytep buf)
{
   /* ANSI-C requires an int value to accommodate at least 16 bits so this
    * works and allows the compiler not to worry about possible narrowing
    * on 32-bit systems.  (Pre-ANSI systems did not make integers smaller
    * than 16 bits either.)
    */
   unsigned int val =
       ((unsigned int)(*buf) << 8) +
       ((unsigned int)(*(buf + 1)));

   return (png_uint_16)val;
}

png_get_uint_16)(png_const_bytep buf)
{
   /* ANSI-C requires an int value to accommodate at least 16 bits so this
    * works and allows the compiler not to worry about possible narrowing
    * on 32-bit systems.  (Pre-ANSI systems did not make integers smaller
    * than 16 bits either.)
    */
   unsigned int val =
       ((unsigned int)(*buf) << 8) +
       ((unsigned int)(*(buf + 1)));

   return (png_uint_16)val;
}
