src/api: Fix ret value in cgroup_get_current_controller_path()

src/api: Fix ret value in cgroup_get_current_controller_path()

The CodeQL reported a warning:
"Incorrect return-value check for a 'scanf'-like function"

More information about the warning:
https://github.com/libcgroup/libcgroup/security/code-scanning/16

Fix the warning by adding an additional "EOF" check for the ret value
from fscanf() in the cgroup_get_current_controller_path()

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