When rte_eth_rx_burst() is called on an memif device with a burst size greater
than 32, there's a segfault in eth_memif_rx() function.

  Thread 1 "x" received signal SIGSEGV, Segmentation fault.
  0x00007ffff7a116d1 in eth_memif_rx (queue=0x1b73051c0, bufs=0x7fffffffd320,
nb_pkts=<optimized out>) at ../drivers/net/memif/rte_eth_memif.c:360
  360                             mbuf = mbuf_head;
  (gdb) bt
  #0  0x00007ffff7a116d1 in eth_memif_rx (queue=0x1b73051c0,
bufs=0x7fffffffd320, nb_pkts=<optimized out>) at
../drivers/net/memif/rte_eth_memif.c:360
  #1  0x000055555555554e in rte_eth_rx_burst (nb_pkts=<optimized out>,
rx_pkts=0x7fffffffd220, queue_id=0, port_id=0) at
/usr/local/include/rte_ethdev.h:5918
  #2  rxloop (queue=0, mp=<optimized out>, burstSize=<optimized out>, port=0)
at x.c:51
  #3  main (argc=<optimized out>, argv=<optimized out>) at x.c:103