Avoid a texture offset if framebuf attach fails

But, I'm not sure in what case this could happen, since `success` should only be true if it attached one (unless something is in `fbCache_` twice?)

Anyway, if there is a case where it's not attached, we should not return true in SetOffsetTexture(), since that will adjust the texturing to account for a framebuffer that is not there.  Although, I'm not sure we're handling that case exactly right still...

@thedax, does #6529 still work with this change?

-[Unknown]