udt: support pointers in auto UDT structs

[mmatczuk]

If there is a nil pointer in an automatic UDT struct code panics.

panic({0xeee920, 0x4000370b10})
        runtime/panic.go:1052 +0x2b4
reflect.flag.mustBeAssignableSlow(0x96)
        reflect/value.go:262 +0xac
reflect.flag.mustBeAssignable(...)
        reflect/value.go:249
reflect.Value.Set({0x121f280, 0x400083d6b8, 0x96}, {0x121f280, 0x4000656b58, 0x16})
        reflect/value.go:1899 +0x34
github.com/scylladb/go-reflectx.FieldByIndexes({0x11c5a80, 0x400083d680, 0x99}, {0x40006a89d8, 0x1, 0x1})
        github.com/scylladb/go-reflectx@v1.0.1/reflect.go:212 +0x15c
github.com/scylladb/go-reflectx.(*Mapper).FieldMap(0x40000a41e0, {0x11c5a80, 0x400083d680, 0x99})
        github.com/scylladb/go-reflectx@v1.0.1/reflect.go:125 +0x1a8
github.com/scylladb/gocqlx/v2.makeUDT(...)
        github.com/scylladb/gocqlx/v2@v2.6.1-0.20211220144210-2b885ac61b11/udt.go:35
github.com/scylladb/gocqlx/v2.udtWrapSlice(0x40000a41e0, 0x0, {0x40001b1700, 0x8, 0x8})
        github.com/scylladb/gocqlx/v2@v2.6.1-0.20211220144210-2b885ac61b11/udt.go:70 +0x158
github.com/scylladb/gocqlx/v2.(*Queryx).Bind(...)
        github.com/scylladb/gocqlx/v2@v2.6.1-0.20211220144210-2b885ac61b11/queryx.go:213
github.com/scylladb/gocqlx/v2.(*Queryx).BindStruct(0x40002e93c0, {0xffde20, 0x40001fcd00})
        github.com/scylladb/gocqlx/v2@v2.6.1-0.20211220144210-2b885ac61b11/queryx.go:128 +0x138

Gocqlx should support fields of pointer type, allocate memory and handle that gracefully.