write_riemann plugin: Terminate `riemann_event_set` arguments with `RIEMANN_EVENT_FIE...

write_riemann plugin: Terminate `riemann_event_set` arguments with `RIEMANN_EVENT_FIELD_NONE`.

`riemann_event_set` is a variadic function, that means it accepts a variable
number of arguments. That means it needs some way to determine – at runtime
– how many arguments there are. It appears to be doing so by using
`RIEMANN_EVENT_FIELD_NONE` to indicate the last element in the argument
list. Unfortunately I was unable to find the library's documentation and
code and could not verify this.

That means that the argument list passed to `riemann_event_set` was not
always terminated, causing it to read past where it was supposed to and
adding random crap into the message it crafted.

Issue: #4050