app/test: fix crash in performance test

app/test: fix crash in performance test

Fix crash in pmd_perf_test autotest (div by 0) when no packets received
Also fixes the fact that the test passes even if exec_burst fails

To repeat the issue:
  The system must be incorrectly set up so that all packets will be lost,
  i.e. no loopback cable, etc. This is an edge case, but still the test
  should not crash or pass when failing.
  run the test app
  RTE>> set_rxtx_sc poll_before_xmit
  RTE>> pmd_perf_autotest
  --snip--
  > Generate 4096 packets @socket 1
  > start to receive total expect 4096
  > 4096 packets lost, IDLE 10000 times
  > Floating point exception (core dumped)

Signed-off-by: David Hunt <david.hunt@intel.com>