Graphics prefix path does not work if there is no subdirectory

Serving CSS images off another domain or subdomain only works if you include a subdirectory in the "Graphics Prefix".

ie:
img.domain.com/images works
img.domain.com doesn't work
img.domain.com/ doesn't work (and tips say not to have a trailing slash anyway)

But if you want to use a subdomain that just points to your main domain's directory structure then you don't want to include a path because the directory structure is identical.

Patch attached.