luci-app-dawn: fix crash issue when viewing hearing map

When viewing the hearing map, sometimes a crash will happen when one of the
routers suddenly cannot view a client. This PR simply checks that the radio
frequency in data2 is not 0, and skips the entry if it is a 0.

Submitted-by: Brian Coverstone <brian@mainsequence.net>
[fix commit subject, add commit description]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>