examples/l2fwd-crypto: fix possible out-of-bounds

examples/l2fwd-crypto: fix possible out-of-bounds

rte_cryptodev_socket_id() returns the socket id of
a crypto device, unless the device id is not valid,
in which case, it returns -1.
This should not happen, as the device id is controlled
by the application, but just for safety, a check is added.

Coverity issue: 158628, 158638, 158656, 158662
Fixes: b3bbd9e5f265 ("cryptodev: support device independent sessions")
Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>