Document validation infinite loop

[radwaretaltr]

Hi,

We found out that in some cases, the Validate() method enters an infinite loop when validating a valid API document.

For example:
https://github.com/APIs-guru/openapi-directory/blob/51fed63e3e6bce022cedba44ce28863bd712040c/APIs/presalytics.io/ooxml/0.1.0/openapi.yaml

This happens because the "stack" slice isn't handled correctly in the validate receiver of "*Schema":
func (schema *Schema) validate(ctx context.Context, stack []*Schema) error

While the slice is being resliced during its lifecycle in the recursion, we do not store the resliced result and use the original slice instead.

NOTE: In order to load this spec, we had to set the "CircularReferenceCounter" to 20.

[radwaretaltr]

Submitted a PR with the fix: #797.

[sambercovici]

@fenollp , can you please confirm the problem and assis?