examples/ipsec-secgw: fix over MTU packet crash

examples/ipsec-secgw: fix over MTU packet crash

When sending an encrypted packet which size after encapsulation exceeds
MTU, ipsec-secgw application tries to fragment it. If --reassemble
option has not been set it results with a segmantation fault, because
fragmentation buckets have not been initialized.

Fix crashing by adding extra check if --ressemble option has not been
set and packet exceeds MTU after encapsulation - drop it.

Fixes: b01d1cd213 ("examples/ipsec-secgw: support fragmentation and reassembly")
Cc: stable@dpdk.org
Signed-off-by: Marcin Smoczynski <marcinx.smoczynski@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Akhil Goyal <akhil.goyal@nxp.com>