WithTransform of nil error panics

[cbandy]

The following panics in v1.10.5:

var err error
gomega.WithTransform(
	func(error) bool { return true },
	gomega.BeTrue(),
).Match(err)

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x56b58b]

goroutine 1 [running]:
github.com/onsi/gomega/matchers.(*WithTransformMatcher).Match(0xc00005e380, 0x0, 0x0, 0x7d5a18, 0xc00005e380, 0xc00006a058)
        /home/cbandy/go/pkg/mod/github.com/onsi/gomega@v1.10.5/matchers/with_transform.go:45 +0x6b
main.main()

I encountered it while using Eventually() to watch for a specific error, e.g.

Eventually(func () error { ... }).Should(WithTransform(os.IsNotExist, BeTrue()))

[ghost]

Looks like WithTransform doesn't handle the actual being nil:

gomega/matchers/with_transform.go

Lines 42 to 45 in c479356

	func (m *WithTransformMatcher) Match(actual interface{}) (bool, error) {
	// return error if actual's type is incompatible with Transform function's argument type
	actualType := reflect.TypeOf(actual)
	if !actualType.AssignableTo(m.transformArgType) {

[onsi]

sounds right - @jim-slattery-rs are you up for and have time to work on a PR to cover this case?

[ghost]

Sure thing, I've created a PR: #422