timer: fix null access if not initialized

If the timer subsystem is not initialized before rte_timer_manage (for
example) is invoked, a pointer to a shared hugepage memory region will
still be null and dereferenced when it is checked for validity; handle
this case.

Fixes: c0749f7096c7 ("timer: allow management in shared memory")
Cc: stable@dpdk.org
Signed-off-by: Erik Gabriel Carrillo <erik.g.carrillo@intel.com>