fix: panics when private field is validated

[ykalchevskiy]

Fixes Or Enhances

dive, omitnil and time-related validations panic by themselves.
panic inside panic happens for most of the checks in case of bad type.

(reflect.Value).Interface() can't be used if a field is private. getValue must be used instead.
In case of casting to string, fmt is smart enough by itself.
In case of getting type with %T, (reflect.Value).Type() can be safely used.

Without these changes tests fail with this error:

panic: reflect.Value.Interface: cannot return value obtained from unexported field or method [recovered]
        panic: reflect.Value.Interface: cannot return value obtained from unexported field or method

goroutine 22 [running]:
testing.tRunner.func1.2({0x10137e740, 0x101416580})
        /Users/yan/.go/src/testing/testing.go:1734 +0x1ac
testing.tRunner.func1()
        /Users/yan/.go/src/testing/testing.go:1737 +0x334
panic({0x10137e740?, 0x101416580?})
        /Users/yan/.go/src/runtime/panic.go:792 +0x124
reflect.valueInterface({0x10137e740?, 0x140002183a0?, 0x10141c850?}, 0xa0?)
        /Users/yan/.go/src/reflect/value.go:1495 +0xc8
reflect.Value.Interface(...)
        /Users/yan/.go/src/reflect/value.go:1484
github.com/go-playground/validator/v10.(*validate).traverseField(0x140001805a0, {0x10141a610, 0x1016cd0a0}, {0x1013ba060?, 0x1400008f550?, 0x17?}, {0x1013a7500?, 0x1400008f550?, 0x10137e980?}, {0x14000190340, ...}, ...)
        /Users/yan/projects/validator/validator.go:315 +0x222c
github.com/go-playground/validator/v10.(*validate).validateStruct(0x140001805a0, {0x10141a610, 0x1016cd0a0}, {0x101377bc0?, 0x1400008f550?, 0x101?}, {0x1013ba060?, 0x1400008f550?, 0x10137e980?}, {0x10141fd28, ...}, ...)
        /Users/yan/projects/validator/validator.go:72 +0x50c
github.com/go-playground/validator/v10.(*Validate).StructCtx(0x140000d03f0, {0x10141a610, 0x1016cd0a0}, {0x101377bc0, 0x1400008f550})
        /Users/yan/projects/validator/validator_instance.go:372 +0x324
github.com/go-playground/validator/v10.(*Validate).Struct(...)
        /Users/yan/projects/validator/validator_instance.go:346
github.com/go-playground/validator/v10.TestPrivateFieldsStruct(0x14000082fc0)
        /Users/yan/projects/validator/validator_test.go:14181 +0x368
testing.tRunner(0x14000082fc0, 0x101413400)
        /Users/yan/.go/src/testing/testing.go:1792 +0xe4
created by testing.(*T).Run in goroutine 1
        /Users/yan/.go/src/testing/testing.go:1851 +0x374
FAIL    github.com/go-playground/validator/v10  1.197s

Make sure that you've checked the boxes below before you submit PR:

• Tests exist or have been written that cover this particular change.

@go-playground/validator-maintainers

[coveralls]

coverage: 73.661% (+0.006%) from 73.655%
when pulling a230cca on ykalchevskiy:master
into 5b31512 on go-playground:master.

[ykalchevskiy]

@deankarn , @nodivbyzero , can you please take a look?

[nodivbyzero]

Hello,

What issue does your PR address?
Based on the provided panic call stack, could you share a specific example that triggers this panic?

[ykalchevskiy]

Hi @nodivbyzero ,

Here you go https://go.dev/play/p/D5c3QKQlnWt
The first and second should simply work.
The 3rd should still panic, but because of the bad type.
I've already added tests for these cases.

There are other cases, basically, (reflect.Value).Interface() is not safe to use with private fields. I've changed all the usages. getValue would just work everywhere, but in some cases there are easier ways, as I've explained in the PR's description.