interaction_id bug for "older" larcv files

[yeonjaej]

v4 file (/sdf/data/neutrino/generic/mpvmpr_2020_01_v04/train.root) interaction_id is not filled properly with below lines.
(np.unique returns [-1, 65535])
When the lines are commented out, the interaction_id seems sensible.

lartpc_mlreco3d/mlreco/utils/particles.py

Lines 69 to 72 in dfee62a

	inter_ids = np.array([p.interaction_id() for p in particles], dtype=np.int32)
	if np.any(inter_ids != INVAL_ID):
	inter_ids[~valid_mask] = -1
	return inter_ids