During tunnel decapsulation the below steps are performed:
 [1] Tunnel information is populated in packet metadata i.e packet->md->tunnel.
 [2] Outer header gets popped.
 [3] Packet is recirculated.

For [1] to work, the dp_packet L3 and L4 header offsets should be valid.
The offsets in the dp_packet are set as part of miniflow extraction.

If offsets are accidentally reset (or) the pop header operation is performed
prior to miniflow extraction, step [1] fails silently and creates
issues that are harder to debug. Add the assertion to check if the
offsets are valid.

Signed-off-by: Bhanuprakash Bodireddy <bhanuprakash.bodireddy@intel.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>