src/collectd-nagios.c: Improve handling of lines returned from `GETVAL'.

src/collectd-nagios.c: Improve handling of lines returned from `GETVAL'.

Then selecting only certain data sources with the `-d' option, the program
would try to read more lines from the server than the server reports, resulting
in the communication never finishing.

This patch resolves that problem and introduces a couple other error handling
blocks.

Thanks to Fabian Linzberger for pointing this out.