fixed panic on pointers to embedded structs

[morus12]

Summary of Changes

 type Example struct {
    *Embedded
     Field string
 }

When a struct contains a pointer to an embedded struct, the code panics because the fields in the embedded struct are not allocated.
Part of the stack trace:

reflect: indirection through nil pointer to embedded struct goroutine 90 [running]:
reflect.Value.FieldByIndex(0x1bbc4e0, 0xc0003e7ec0, 0x199, 0xc0002c6e40, 0x2, 0x2, 0x0, 0x1d93dc0, 0x1aeb9c0)
	/usr/local/go/src/reflect/value.go:876 +0x4f2
reflect.Value.FieldByName(0x1bbc4e0, 0xc0003e7ec0, 0x199, 0x1b54e80, 0x4, 0x199, 0x15afec3, 0xc0003e7eba)
	/usr/local/go/src/reflect/value.go:892 +0x116
github.com/gorilla/schema.(*Decoder).decode(0xc0002b9e40, 0x1b36ae0, 0xc0000d8280, 0x196, 0xc0004d0799, 0xc, 0xc0000e6ca8, 0x1, 0x1, 0xc0002b9e60, ...)
	/Users/grzegorzwyszyski/Workspace/gopath/pkg/mod/github.com/gorilla/schema@v1.1.0/decoder.go:185 +0x212
github.com/gorilla/schema.(*Decoder).decode(0xc0002b9e40, 0x1bf0600, 0xc00032e1e0, 0x199, 0xc0004d0799, 0xc, 0xc0000e6c80, 0x2, 0x2, 0xc0002b9e60, ...)
	/Users/grzegorzwyszyski/Workspace/gopath/pkg/mod/github.com/gorilla/schema@v1.1.0/decoder.go:213 +0x28aa
github.com/gorilla/schema.(*Decoder).Decode(0xc0002b9e40, 0x1b36a60, 0xc00032e1e0, 0xc00021f200, 0x2, 0x3b)
	/Users/grzegorzwyszyski/Workspace/gopath/pkg/mod/github.com/gorilla/schema@v1.1.0/decoder.go:80 +0x3c6

Panic can be reproduced using the test provided by this pull request.

[stale]

This issue has been automatically marked as stale because it hasn't seen a recent update. It'll be automatically closed in a few days.

[elithrar]

Thanks @morus12