Initialize received variable. Otherwise, is is possible for server to
answer without any contexts, but we will set context_id to something
random (received_id is not initialized too) and return 1, which is
wrong.

To solve it, just initialize received to false. Initialize received_id
too, just to make all possible checkers happy.

Bug was introduced in 78a33ab5 "nbd: BLOCK_STATUS for
standard get_block_status function: client part" with the whole
function.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20180427142002.21930-2-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
CC: qemu-stable@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>