kvargs: fix processing a null list

kvargs: fix processing a null list

In the doxygen description of rte_kvargs_process(), it is said:
	If *kvlist* is NULL function does nothing.
It has been added by mistake here instead of rte_kvargs_free().
Anyway, null list should be correctly handled in both functions.

Comments are fixed in both functions and NULL handling is added
to rte_kvargs_process().

Fixes: c34af7424e09 ("kvargs: fix freeing behaviour for null")
Cc: stable@dpdk.org
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Olivier Matz <olivier.matz@6wind.com>