Bluetooth: Controller: Workaround datapath field access after release

Workaround the access that check if datapath is NULL when
the stream has already been released. Memory Pool overwrites
first 8 bytes to maintain the free list and free count. This
causes the datapath fields if placed in the first 8 bytes to
not be NULL after the memory is freed.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>