Ignore invalid LAC

[Helium314]

I noticed some bad GSM-based location reports with LAC (location area code) 0.
However, at least for GSM cells (maybe others as well), a valid LAC can't be 0, see

Location Area Code (LAC) which is a fixed length code (of 2 octets) identifying a location area within a GSM PLMN. This part of the location area identification can be coded using a full hexadecimal representation except for the following reserved hexadecimal values:
0000, and
FFFE.

from https://www.etsi.org/deliver/etsi_ts/123000_123099/123003/03.07.00_60/ts_123003v030700p.pdf

Proposed solution:
Treat LAC 0 as invalid.

(related to #33)