api.c: prevent array out-of-bounds access in cgroup_create_template_group

api.c: prevent array out-of-bounds access in cgroup_create_template_group

In the function src/api.c/cgroup_create_template_group,
the loop condition:

while (tmp->controllers[i] != NULL) {

allows accessing tmp->controllers[MAX_MNT_ELEMENTS] if tmp->controllers
is full and lacks a terminating NULL.

Add explicit bounds checking (i < MAX_MNT_ELEMENTS) while maintaining
the NULL check. This ensures that there will never be reading past
the array boundaries regardless of its content.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Signed-off-by: Mikhail Dmitrichenko <m.dmitrichenko222@gmail.com>
Acked-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>