test/ring: fix build for O1 optimization

test/ring: fix build for O1 optimization

When DPDK is compiled with gcc < 9 with the optimization level set to 1
gcc sees zcd in test_ring.h as possibly being uninitialised. To correct
this error if statements from _st_ring_dequeue_bulk and
_st_ring_enqueue_bulk were corrected within test_ring_mt_peek_stress_zc.c

Fixes: f72299fd157d ("test/ring: add stress tests for zero copy API")

Signed-off-by: Conor Walsh <conor.walsh@intel.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>