fix: invalid pointer deref in pgotel instrumentation

[fernandez14]

We've been experiencing panics on production code with a stack trace like this:

panic runtime error: invalid memory address or nil pointer dereference [recovered] 
    go.opentelemetry.io/otel/sdk/trace/span.go:383 (*recordingSpan).End.func1
    go.opentelemetry.io/otel/sdk/trace/span.go:421 (*recordingSpan).End
    /usr/local/go/src/runtime/panic.go:914 panic
    github.com/go-pg/pg/v10/result.go:48 (*result).RowsAffected
    github.com/go-pg/pg/extra/pgotel/v10/pgotel.go:129 (*TracingHook).AfterQuery
    github.com/go-pg/pg/v10/hook.go:130 (*baseDB).afterQueryFromIndex
    github.com/go-pg/pg/v10/hook.go:125 (*baseDB).afterQuery
    github.com/go-pg/pg/v10/tx.go:238 (*Tx).query
    github.com/go-pg/pg/v10/tx.go:211 (*Tx).QueryContext
    github.com/go-pg/pg/v10/orm/query.go:1163 (*Query).returningQuery
    github.com/go-pg/pg/v10/orm/query.go:1034 (*Query).Insert

We noticed that, while there's a nil guard on evt.Result, we also need to check that, when result implementation is filled with a pointer that pointer is not nil.

I've written a similar example of this issue on https://go.dev/play/p/mVBQkMv7lJU

[elliotcourant]

Would you be able to add a test similar to the one you have in that go playground as part of this PR? That way it's easier to make sure this doesn't break again in the future

[fernandez14]

Would you be able to add a test similar to the one you have in that go playground as part of this PR? That way it's easier to make sure this doesn't break again in the future

I isolated the check into a function and added a test to it. Let me know how it looks and thanks for taking a look.

[elliotcourant]

@fernandez14 sorry for the delay but your change is now tagged in v10.12.0. Thank you again!