mm: swap: Add null pointer check

Address an unlikely null pointer dereference.
Fixes commit 7b5df13b ("mm: swap: free up swap on mm reap").

The expected usage of free_swap_slot is that the caller has already
checked that the swap_entry_t is valid.

Change-Id: I85331f817458df7f6cb0c0b387ca34c345f8c513
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>