Malformed flame graph when importing delta heap profile from Go

It has been reported that importing a delta heap profile from Go in the ad-hoc view results in a malformed flame graph. It’s worth noting that the pprof tool does not handle this properly either. Apparently, samples with negative values are not sanitized.

A delta profile can be collected using the standard Go pprof HTTP endpoint:
```
curl http://127.0.0.1:12345/debug/pprof/heap?seconds=30 > heap.pprof
```

The issue affects `inuse_*` samples the most.