Issues with datatypes and missing errorhandling

[denzs]

Hi there, first let me say, awesome project!! :)

While playing with the Dockerimage i found that the metrics from core.shmmem are not exported correclty.. They were '0' all the time.

While diving into the code i found, that potential errors from the typecasting calls in the switch statement in https://github.com/florentchauveau/kamailio_exporter/blob/master/collector.go#L419 are not considered..

After adding logmessages for that i was able to workaround the issue with core.shmmem (moved position in switch statement to use ..Double() instead of Int()).

I can make a PR with my changes, but i am not sure if it contains the best solution..

I think it would be best, to have a generic wrapper inside of https://github.com/florentchauveau/go-kamailio-binrpc/blob/master/binrpc.go instead of calling the correct function.. but that overruns my knowledge ;)