gcc -Wall warns about uninitialized variables (-Wmaybe-uninitialized), and
-Werror makes it fatal. This change allows the build to succeed by NULL'ifying
the pointer passed to strtok_r().

Note that strtok_r(3) anyway ignores a non-NULL arg3 pointer on the 1st call
with non-NULL arg1 string.
Signed-off-by: Leonid Isaev <leonid.isaev@jila.colorado.edu>
Acked-by: Serge Hallyn <serge.hallyn@ubuntu.com>