cgcreate: fix null pointer dereference in create_systemd_scope()

cgcreate: fix null pointer dereference in create_systemd_scope()

Fix a NULL pointer dereference, reported by the Coverity tool:

CID 321267 (#1 of 1): Dereference null return value (NULL_RETURNS)6.
dereference: Dereferencing a pointer that might be NULL scope when
calling strlen.

check for the return value from strstr(), before dereferencing it.

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