kni: fix possible uninitialized variable

kni: fix possible uninitialized variable

This error can be raised:
	lib/librte_kni/rte_kni.c:531:15: error:
	'req' may be used uninitialized in this function

It should not happen because kni_fifo_get() would return 0 if
req is not initialized, so the function would return before using req.
But GCC complains about it in -O1 optimization,
and a NULL initialization is harmless here.

Fixes: 3fc5ca2f6352 ("kni: initial import")
Cc: stable@dpdk.org
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>