FOF with hydro zooms

Looks like there's a possible issue when FOF is performed on zooms over MPI.

There are some reallocations of memory that can be zero which crash the code (from a node empty of zoom region particles).

Changing the reallocs in lines 1869, 1707 and 1083 to include a "+ 1" in fof.c seems to fix the issue, but not sure if this is the best solution.