Fix nil pointer dereference in ReflectFromType with ExpandedStruct (fix #163)

[edznux-dd]

This PR fixes #163

I'm not 100% sure if this is still maintained, but I stumbled uppon what I believe is a bug or at least an unexpected behavior.
The impact for my app was fairly low (dev time), but unexpected.

Example reproducer based on your documentation example:

package main

import (
	"encoding/json"
	"fmt"
	"reflect"
	"time"

	"github.com/invopop/jsonschema"
)

type SampleUser struct {
	ID int `json:"id"`
}

func main() {
	r := &jsonschema.Reflector{ExpandedStruct: true}

	// The following line works fine (based on the example from the doc)
	// s := r.ReflectFromType(reflect.TypeOf(SampleUser{}))

	// The following line panic (only if ExpandedStruct: true)
	s := r.ReflectFromType(reflect.TypeOf(map[string]interface{}{}))

	data, err := json.MarshalIndent(s, "", "  ")
	if err != nil {
		panic(err.Error())
	}
	fmt.Println(string(data))
}

The fix is "just" checking that the key we are deleting exists.

I've added a fuzz test in addition of the regression test, in case we want to discover more bugs in the future (and possibly let agent fix them for you!). The fuzz-test, tests a few more flags combination to have more code coverage.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.34%. Comparing base (d39f13c) to head (b43264d).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #186      +/-   ##
==========================================
+ Coverage   89.30%   89.34%   +0.04%     
==========================================
  Files           4        4              
  Lines         729      732       +3     
==========================================
+ Hits          651      654       +3     
  Misses         59       59              
  Partials       19       19              

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[samlown]

Thanks for this! A few linter issues that I'll have a look at.

[samlown]

I'm not 100% sure if this is still maintained

We try, and actively use this in production, its just that time is very limited. Well tested PRs like this are great.