fix realloc() error on reboot container

The container with "lxc.network.type=phys" halted with error on reboot.

Error message:
*** glibc detected *** lxc-start: realloc(): invalid pointer: 0x0948eed0 ***

We have a sequence:

1) conf->saved_nic = relloc(NULL) on start start.c:container save_phys_nics()
2) free(conf->saved_nics) after stop container
   conf.c:lxc_rename_phys_nics_on_shutdown()
3) conf->saved_nic = relloc(conf->saved_nics) on restart container
   start.c:save_phys_nics() -> error relloc()

free(conf->saved_nics) in lxc_rename_phys_nics_on_shutdown()
unnecessary, it will be called later in lxc_clear_saved_nics().

Signed-off-by: Vitaly Lavrov <vel21ripn@gmail.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
bdev: do not crash if specs is NULL

Signed-off-by: S.Çağlar Onur <caglar@10ur.org>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>