fix nil pointer dereference when no callback passed to provider

[robinbraemer]

When doing

k.Load(env.Provider("", ".", nil), nil)

koanf always wraps the nil callback cb

koanf/providers/env/env.go

Lines 33 to 41 in 5f508c9

	func Provider(prefix, delim string, cb func(s string) string) *Env {
	return &Env{
	prefix: prefix,
	delim: delim,
	cb: func(key string, value string) (string, interface{}) {
	return cb(key), value
	},
	}
	}

and would panic because

koanf/providers/env/env.go

Line 81 in 5f508c9

if e.cb != nil {

is always true but the wrapped cb is nil.

panic: runtime error: invalid memory address or nil pointer dereference [recovered]
        panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x10ae7a7]

goroutine 6 [running]:
testing.tRunner.func1.2({0x14e6300, 0x24c6d40})
        /home/robin/go/go1.17.1/src/testing/testing.go:1209 +0x24e
testing.tRunner.func1()
        /home/robin/go/go1.17.1/src/testing/testing.go:1212 +0x218
panic({0x14e6300, 0x24c6d40})
        /home/robin/go/go1.17.1/src/runtime/panic.go:1038 +0x215
github.com/knadh/koanf/providers/env.Provider.func1({0xc000040000, 0x19}, {0xc000040015, 0x4})
        /home/robin/go/pkg/mod/github.com/knadh/koanf@v1.3.3/providers/env/env.go:38 +0x27
github.com/knadh/koanf/providers/env.(*Env).Read(0xc0002c0270)
        /home/robin/go/pkg/mod/github.com/knadh/koanf@v1.3.3/providers/env/env.go:82 +0x1a3
github.com/knadh/koanf.(*Koanf).Load(0x0, {0x191c880, 0xc0002c0270}, {0x0, 0x0})
        /home/robin/go/pkg/mod/github.com/knadh/koanf@v1.3.3/koanf.go:101 +0x9e