fix: panic when Decode's input is array and output is a slice

[suzuki-shunsuke]

Close #264

Test

I have confirmed the issue is reproduced by 74cdd45

$ go test -race -run Decode_decodeSliceWithArray ./...
--- FAIL: TestDecode_decodeSliceWithArray (0.00s)
    --- FAIL: TestDecode_decodeSliceWithArray/input_is_array (0.00s)
panic: reflect: call of reflect.Value.IsNil on array Value [recovered]
	panic: reflect: call of reflect.Value.IsNil on array Value

goroutine 20 [running]:
testing.tRunner.func1.2({0x1237e60, 0xc0001b21b0})
	/usr/local/Cellar/go/1.17.5/libexec/src/testing/testing.go:1209 +0x36c
testing.tRunner.func1()
	/usr/local/Cellar/go/1.17.5/libexec/src/testing/testing.go:1212 +0x3b6
panic({0x1237e60, 0xc0001b21b0})
	/usr/local/Cellar/go/1.17.5/libexec/src/runtime/panic.go:1047 +0x266
reflect.Value.IsNil(...)
	/usr/local/Cellar/go/1.17.5/libexec/src/reflect/value.go:1427
github.com/mitchellh/mapstructure.(*Decoder).decodeSlice(0xc0001a4048, {0x0, 0x0}, {0x1233220, 0x13b9f68}, {0x1230140, 0xc0001b2180, 0x9ca1ed8})
	/Users/shunsuke-suzuki/repos/src/github.com/mitchellh/mapstructure/mapstructure.go:1092 +0xfd1
github.com/mitchellh/mapstructure.(*Decoder).decode(0xc0001a4048, {0x0, 0x2880008}, {0x1233220, 0x13b9f68}, {0x1230140, 0xc0001b2180, 0x10aa9bb})
	/Users/shunsuke-suzuki/repos/src/github.com/mitchellh/mapstructure/mapstructure.go:469 +0x815
github.com/mitchellh/mapstructure.(*Decoder).decodePtr(0xc0001a4048, {0x0, 0x0}, {0x1233220, 0x13b9f68}, {0xc0001e06c0, 0xc0001b2180, 0x1230140})
	/Users/shunsuke-suzuki/repos/src/github.com/mitchellh/mapstructure/mapstructure.go:1031 +0x73b
github.com/mitchellh/mapstructure.(*Decoder).decode(0xc0001a4048, {0x0, 0x8}, {0x1233220, 0x13b9f68}, {0xc0001e06c0, 0xc0001b2180, 0xc0001a62a0})
	/Users/shunsuke-suzuki/repos/src/github.com/mitchellh/mapstructure/mapstructure.go:467 +0x630
github.com/mitchellh/mapstructure.(*Decoder).decodeBasic(0x1078872, {0x0, 0x0}, {0x1233220, 0x13b9f68}, {0x1236aa0, 0xc0001e06a0, 0x12313e0})
	/Users/shunsuke-suzuki/repos/src/github.com/mitchellh/mapstructure/mapstructure.go:513 +0x63f
github.com/mitchellh/mapstructure.(*Decoder).decode(0xc0001a4048, {0x0, 0x3636000009ca2f50}, {0x1233220, 0x13b9f68}, {0x1236aa0, 0xc0001e06a0, 0xc00018e1d0})
	/Users/shunsuke-suzuki/repos/src/github.com/mitchellh/mapstructure/mapstructure.go:453 +0x67b
github.com/mitchellh/mapstructure.(*Decoder).Decode(0xc0001a4048, {0x1233220, 0x13b9f68})
	/Users/shunsuke-suzuki/repos/src/github.com/mitchellh/mapstructure/mapstructure.go:398 +0x1dd
github.com/mitchellh/mapstructure.Decode({0x1233220, 0x13b9f68}, {0x122cdc0, 0xc0001e06a0})
	/Users/shunsuke-suzuki/repos/src/github.com/mitchellh/mapstructure/mapstructure.go:302 +0x151
github.com/mitchellh/mapstructure.TestDecode_decodeSliceWithArray.func1(0xc000183a00)
	/Users/shunsuke-suzuki/repos/src/github.com/mitchellh/mapstructure/mapstructure_test.go:580 +0x87
testing.tRunner(0xc000183a00, 0xc00019ad60)
	/usr/local/Cellar/go/1.17.5/libexec/src/testing/testing.go:1259 +0x230
created by testing.(*T).Run
	/usr/local/Cellar/go/1.17.5/libexec/src/testing/testing.go:1306 +0x727
FAIL	github.com/mitchellh/mapstructure	0.246s
FAIL

And the issue is solved by ab94595

$ go test -race -run Decode_decodeSliceWithArray ./...
ok  	github.com/mitchellh/mapstructure	0.245s

[suzuki-shunsuke]

https://pkg.go.dev/reflect#Value.IsNil

The argument must be a chan, func, interface, map, pointer, or slice value; if it is not, IsNil panics.

[suzuki-shunsuke]

I'm sorry but I don't know how to add tests in this repository.
If this isn't good, please let me know.
I'll fix.

[mitchellh]

Thank you!