wrapper: fix segfault in cgroup_set_value_bool()

wrapper: fix segfault in cgroup_set_value_bool()

The second argument passed to cgroup_set_value_bool() is of type
char * and the user might pass NULL in the place of the argument,
causing a segfault.  The reason is, argument values are used without
checks, fix it by checking for NULL pointers before proceeding.

Reproducer:
----------

 #include <stdlib.h>
 #include <libcgroup.h>

 int main(void)
 {
 	struct cgroup_controller *cgc;
 	struct cgroup *cgrp;
 	int ret;

 	ret = cgroup_init();
 	if (ret)
 		exit(1);

 	cgrp = cgroup_new_cgroup("fuzzer");
 	if (!cgrp)
 		exit(1);

 	cgc = cgroup_add_controller(cgrp, "cpuset");
 	if (!cgc)
 		exit(1);

 	ret = cgroup_add_value_string(cgc, "cpuset.cpu_exclusive", "0");
 	if (ret)
 		exit (1);

 	cgroup_set_value_bool(cgc, NULL, 0);
 	//should not reach here
 	return 0;
 }

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>