CGContextDrawImage(context, rect

在截取头像时，需要获取一张图片的指定rect内的图片，如果传入的rect在原图片之外那么就会出现截取的图片拉伸的情况，所以这里rect要做边界判断:

- (UIImage *)thumbImageFromImage:(UIImage *)image imRect:(CGRect)rect {

    if (![image isKindOfClass:[UIImage class]]) {
        return nil;
    }
    CGImageRef imageRef = image.CGImage;
    CGImageRef subImageRef = CGImageCreateWithImageInRect(imageRef, rect);

    UIGraphicsBeginImageContext(rect.size);
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextDrawImage(context, rect, subImageRef);
    UIImage* smallImage = [UIImage imageWithCGImage:subImageRef];
    UIGraphicsEndImageContext();

    CGImageRelease(subImageRef);
    return smallImage;
}