iwinfo: zero out ccode buffer since library doesn't

iwinfo: zero out ccode buffer since library doesn't

Making an ubus call iwinfo countrylist '{"device":"radio0"}' will result
in some entries having garbage uninitialized stack bytes in the "code"
fields.

With this patch we zero-initialize the buffer that libiwinfo writes to,
making it NUL-terminated so that behavior doesn't happen anymore.

Signed-off-by: Denis Osvald <denis.osvald@sartura.hr>