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>

lib/librte_kni/rte_kni.c

@@ -483,7 +483,7 @@ int
 rte_kni_handle_request(struct rte_kni *kni)
 {
 	unsigned ret;
-	struct rte_kni_request *req;
+	struct rte_kni_request *req = NULL;
 
 	if (kni == NULL)
 		return -1;