BuildV3Model returns a partially-built model that panics on public API

[AtanasovskiPetar]

Version: reproduced on v0.38.5 and v0.38.7

If one operation in a path item fails to build, BuildV3Model still returns a model along with the error. The other operations in that path item never get built, so they come back as empty low Operation structs with a nil embedded *low.Reference.

Calling IsReference() on one of those panics with a nil pointer dereference, so one bad $ref can take down anything that walks the model. The $ref error itself is fine, it's the state of the model returned with it that's the problem.

Different from #585.

Reproducer — two operations, only get has the dangling $ref

openapi: 3.0.2
info: { title: t, version: 1.0.0 }
paths:
  /p:
    get:
      responses:
        '200':
          description: ok
          content:
            application/json:
              schema: { $ref: '#/components/schemas/Nope' }
    post:
      responses:
        '200': { description: ok }

model, errs := doc.BuildV3Model()
pi, _ := model.Model.Paths.PathItems.Get("/p")

pi.Get.GoLow().RootNode == nil
pi.Post.GoLow().RootNode == nil
pi.Post.GoLow().IsReference()       // panic: nil pointer dereference

It needs two or more operations in the same path item — a single-operation path item does not reproduce it.

We hit this through daveshanley/vacuum, which walks the model with pb33f/doctor; it panics at doctor/model/high/v3/operation.go:261.