added null check for paramRef when scanning operation params.

this issue was discovered in https://github.com/daveshanley/vacuum/issues/240, this provides a resolution to the problem.

index/utility_methods.go

@@ -302,9 +302,11 @@ func (index *SpecIndex) scanOperationParams(params []*yaml.Node, pathItemNode *y
                     Path: path,
                 })
             } else {
+                if paramRef != nil {
                     index.paramOpRefs[pathItemNode.Value][method][paramRefName] =
                         append(index.paramOpRefs[pathItemNode.Value][method][paramRefName], paramRef)
                 }
+            }
 
             continue