datapath: Add NULL check for mask pointer.

There is race in datapath when last mask in mask array deleted can
result in NULL pointer dereference in datapath.
datapath lookup does not check mask pointer if its index is less than
mask-array count. That is safe because delete operation moves last valid
pointer to the deleted element. But this does not work if we are
deleting last element in array. Following patch adds NULL check for the
mask pointer.
This patch also avoids accessing ma->count without any locks.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Andy Zhou <azhou@nicira.com>