Revert "openvswitch: potential NULL deref in sample()"

This reverts commit 5b3e7e6c.

The problem that the original commit was attempting to fix can
never happen in practice because validation is done one a per-flow
basis rather than a per-packet basis.  Adding additional checks at
runtime is unnecessary and inconsistent with the rest of the code.

CC: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>