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>

applications/luci-app-dawn/luasrc/model/cbi/dawn/dawn_hearing_map.lua

@@ -38,6 +38,7 @@ function s.render(self, sid)
 							local mac2, data2
 							local count_loop = 0
 							for mac2, data2 in pairs(data) do
+								if data2.freq ~= 0 then --prevent empty entry crashes
 					%>
 						<tr class="tr">
 							<td class="td"><%= (count_loop == 0) and mac or "" %></td>
@@ -56,6 +57,7 @@ function s.render(self, sid)
 									count_loop = count_loop + 1
 								end
 							end
+						end
 					%>
 				</table>
 			</div>