examples/performance-thread: fix build with low core count

examples/performance-thread: fix build with low core count

When the value of RTE_MAX_LCORE is small, it results in the
following compilation error.

../examples/performance-thread/l3fwd-thread/main.c:2338:34: error:
iteration 4 invokes undefined behavior
[-Werror=aggressive-loop-optimizations]

Fixes: d48415e1fee3 ("examples/performance-thread: add l3fwd-thread app")
Cc: stable@dpdk.org
Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Tested-by: Lukasz Wojciechowski <l.wojciechow@partner.samsung.com>
Reviewed-by: Lukasz Wojciechowski <l.wojciechow@partner.samsung.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>