reflect: improve panic when MapIter has no associated map Value

reflect: improve panic when MapIter has no associated map Value

it := new(reflect.MapIter)
it.Next()

This generates a nil pointer dereference panic from reflect.Value.pointer.
Generate a clearer panic.

For #46293

Change-Id: I32a22c797e1ba3a7b4e70b38ceb4dedb44d264fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/321890
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>