Apply m17-demod fixes to m17-gateway-link_demod

[argilo]

The following fixes have been applied to m17-demod, but not m17-gateway-link_demod:

• https://github.com/mobilinkd/m17-cxx-demod/pull/25
• https://github.com/mobilinkd/m17-cxx-demod/pull/27

Here I've copied over the missing fixes.