png_read_buffer(png_structrp png_ptr, png_alloc_size_t new_size, int warn)
{
   png_bytep buffer = png_ptr->read_buffer;

   if (buffer != NULL && new_size > png_ptr->read_buffer_size)
   {
      png_ptr->read_buffer = NULL;
      png_ptr->read_buffer = NULL;
      png_ptr->read_buffer_size = 0;
      png_free(png_ptr, buffer);
      buffer = NULL;
   }

   if (buffer == NULL)
   {
      buffer = png_voidcast(png_bytep, png_malloc_base(png_ptr, new_size));

      if (buffer != NULL)
      {
         memset(buffer, 0, new_size); /* just in case */
         png_ptr->read_buffer = buffer;
         png_ptr->read_buffer_size = new_size;
      }

      else if (warn < 2) /* else silent */
      {
         if (warn != 0)
             png_chunk_warning(png_ptr, "insufficient memory to read chunk");

         else
             png_chunk_error(png_ptr, "insufficient memory to read chunk");
      }
   }

   return buffer;
}

The product performs a calculation that can produce an integer overflow or wraparound when the logic assumes that the resulting value will always be larger than the original value. This occurs when an integer value is incremented to a value that is too large to store in the associated representation. When this occurs, the value may become a very small or negative number.