继续看文档，png_create_write_struct_2()这个函数是自己分配png处理内存的方法，不是写内存的初始化,正确的接口是：
png_init_io()初始化读取或写入文件，这是默认方式。
也可以使用 png_set_read_fn
() and png_set_write_fn() 来自定义读写方式。

typedef struct {
    char * data;
    size_t size;
    size_t offset;
}ImageSource;

//写入数据函数，将所有的数据写入缓冲区
void PNGAPI png_own_write_data(png_structp png_ptr, png_bytep data, png_size_t length)
{
    ImageSource* isource = (ImageSource*)png_get_io_ptr(png_ptr);
    if(isource->offset + length <= isource->size)
    {
       memcpy(isource->data + isource->offset,data,  length);
       isource->offset += length;
    }
    else
    {
        png_error(png_ptr,"pngReaderCallback failed");
    }
}
static bool png_creat(AVFrame * pFrame,char * png_buf,size_t date_size)
{
    png_structp png_ptr;
    png_infop info_ptr;
    png_colorp palette;
    int width=pFrame->width,height=pFrame->height;

    png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
    if (png_ptr == NULL)
    {
       return false;
    }

    /* Allocate/initialize the image information data.  REQUIRED */
    info_ptr = png_create_info_struct(png_ptr);
    if (info_ptr == NULL)
    {
        png_destroy_write_struct(&png_ptr,  NULL);
        return false;
    }
    if (setjmp(png_jmpbuf(png_ptr)))
    {
       /* If we get here, we had a problem writing the file */
       png_destroy_write_struct(&png_ptr, &info_ptr);
       return false;
    }
    ImageSource imgsource;
    imgsource.data = png_buf;
    imgsource.size = date_size;
    imgsource.offset = 0;
    png_set_write_fn(png_ptr,&imgsource,png_own_write_data,NULL);


    /* 设置png文件的属性 */
    png_set_IHDR(png_ptr, info_ptr, width, height, 8,
                    PNG_COLOR_TYPE_RGB,
                    PNG_INTERLACE_NONE,
                    PNG_COMPRESSION_TYPE_BASE,
                    PNG_FILTER_TYPE_BASE);

    /* 分配调色板空间。常数 PNG_MAX_PALETTE_LENGTH 的值是256 */
    palette = (png_colorp)png_malloc(png_ptr, PNG_MAX_PALETTE_LENGTH * sizeof(png_color));

    png_set_PLTE(png_ptr, info_ptr, palette, PNG_MAX_PALETTE_LENGTH);

       /* Write the file header information.  REQUIRED */
    png_write_info(png_ptr, info_ptr);

    /* The easiest way to write the image (you may have a different memory
    * layout, however, so choose what fits your needs best).  You need to
    * use the first method if you aren't handling interlacing yourself.
    */
    png_uint_32 k;
    png_byte *image;
    png_bytep row_pointers[height];

    image = pFrame->data[0];

    if (height > PNG_UINT_32_MAX/sizeof(png_bytep))
        png_error (png_ptr, "Image is too tall to process in memory");

    for (k = 0; k < height; k++)
        row_pointers[k] = image + k*width*3;

    /* One of the following output methods is REQUIRED */
    png_write_image(png_ptr, row_pointers);

    //end，进行必要的扫尾工作：
    png_write_end(png_ptr, info_ptr);
    png_free(png_ptr, palette);
    png_destroy_write_struct(&png_ptr, &info_ptr);

}