examples/ipsec-secgw: fix corner case for SPI value

examples/ipsec-secgw: fix corner case for SPI value

IPSec application is using index 0 of SA table as error,
with current value of IPSEC_SA_MAX_ENTRIES(128) it can
not support SA with spi = 128, as it uses sa_idx = 0
in the SA table.

With this patch, sa_idx = 0 can also be used.

PS: spi = 0 is an invalid SPI and application throws error
for it.

Fixes: d299106e8e31 ("examples/ipsec-secgw: add IPsec sample application")
Cc: stable@dpdk.org
Signed-off-by: Akhil Goyal <akhil.goyal@nxp.com>
Acked-by: Radu Nicolau <radu.nicolau@intel.com>