Fix mac_age process crash in multi-threaded environment

VPP crash is observed when MAC aging is enabled with multi-threaded mode.
If a thread other-than-zero expands the working_copies vector,
working_copy_lengths should be initialized with vec_validate_init_empty(..., -1)
to fill -1 across lower-numbered working_copy_lengths vector element.

Change-Id: I60959fc6511306b33acae323df9c6898fc6c50ce
Signed-off-by: Steve Shin <jonshin@cisco.com>