kvargs: fix crash when parsing an invalid token on freebsd

kvargs: fix crash when parsing an invalid token on freebsd

The behavior of strtok_r() is not the same between GNU libc and FreeBSD
libc: in the first case, the context is set to "" when the last token is
returned, while in the second case it is set to NULL.

On FreeBSD, the current code crashes because we are dereferencing a NULL
pointer (ctx1). Fix it by first checking if it is NULL. This works with
both GNU and FreeBSD libc.

Fixes: ffcf831454a9 ("kvargs: fix buffer overflow when parsing list")
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>