Don't reallocate peers-array nor set it to %NULL. Instead,
just emit the signal for the peers and take them out one-by-one.

I am slightly surprised, that the peers array does not need to hold
a reference on the NMP2PPeer instances. But that seems intentional.
I think, the libnm code here should be significantly reworked, but
that is for another time.

Also, delay clearing the pointers until finalize() method. For
the most part, it shouldn't make a difference. Still avoid having
the instance in a badly defined state during dispose() (which
theoretically could be called multiple times).