Validate negative sampling

Lines with negative sampling make server crash.

```
metric_name:43|ms|@-1.000000
```

will cause crash at `stats.js:240`:

```
sampleRate = Number(fields[2].match(/^@([\d\.]+)/)[1]);
```

because there is no match for `([\d\.]+)` group. Solution is to add additional validation against negative numbers in `helpers.js`.