raw/skeleton: fix empty devargs parsing

[ upstream commit 11da6149224a8de53d2ddd2aacba7b158cc4e3b4 ]

The rte_kvargs_process() was used to parse KV pairs, it also supports
to parse 'only keys' (e.g. socket_id) type. And the callback function
parameter 'value' is NULL when parsed 'only keys'.

This patch fixes segment fault when parse input args with 'only keys'
(e.g. self_test).

Fixes: 55ca1b0 ("raw/skeleton: add test cases")

Signed-off-by: Chengwen Feng <fengchengwen@huawei.com>

drivers/raw/skeleton/skeleton_rawdev.c

@@ -659,6 +659,8 @@ skeldev_get_selftest(const char *key __rte_unused,
 		     void *opaque)
 {
 	int *flag = opaque;
+	if (value == NULL || opaque == NULL)
+		return -EINVAL;
 	*flag = atoi(value);
 	return 0;
 }