spapr: Fix undefined behaviour in spapr_tce_reset()

When a TCE table (sPAPR IOMMU context) is in disabled state (which is true
by default for the 64-bit window), it has tcet->nb_table == 0 and
tcet->table == NULL.  However, on system reset, spapr_tce_reset() executes,
which unconditionally calls
        memset(tcet->table, 0, table_size);

We get away with this in practice, because it's a zero length memset(),
but memset() on a NULL pointer is undefined behaviour, so we should not
call it in this case.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
