[igraph] spinglass.community bug

[uxzmdlj02]

I think I've found a tiny but important bug in the spinglass.community 
() function in R.
The argument is passed into the R function as `graph`, but in the  
function the following code is called:

            weights <- as.numeric(E(g)$weight)

That should be

            weights <- as.numeric(E(graph)$weight)

Otherwise it looks for `g` in the parent environment, which is bad.

The change makes the function work for me.

Thanks,
Peter