fusion: Explicitly check for -1 for a missing table

[Robert Shearman]

James Hawkins wrote:
> It works just fine.  -1 is 4294967295 in ULONG (32bit), which is
> exactly the same as offset on error (because we assigned it -1, but
> the representation in memory is the same).

Then use ~0 so that the purpose is clearer. Adding a define for this 
value would probably further increase readability of the code.

> We do this several other
> places in the code base.

They should be changed too.

-- 
Rob Shearman