reflect: Use Value.Len instead of conversion to slice header.

This change is functionally equivalent, but reduces reliance on unsafe
features. This would allow GopherJS to avoid an additional patch to the
standard library we'd have to maintain in order to remain compatible
with Go 1.17+.

Author: nevkontakte

src/reflect/value.go

@@ -2940,8 +2940,7 @@ func (v Value) CanConvert(t Type) bool {
 	// from slice to pointer-to-array.
 	if vt.Kind() == Slice && t.Kind() == Ptr && t.Elem().Kind() == Array {
 		n := t.Elem().Len()
-		h := (*unsafeheader.Slice)(v.ptr)
-		if n > h.Len {
+		if n > v.Len() {
 			return false
 		}
 	}
@@ -3208,10 +3207,10 @@ func cvtStringRunes(v Value, t Type) Value {
 // convertOp: []T -> *[N]T
 func cvtSliceArrayPtr(v Value, t Type) Value {
 	n := t.Elem().Len()
-	h := (*unsafeheader.Slice)(v.ptr)
-	if n > h.Len {
-		panic("reflect: cannot convert slice with length " + itoa.Itoa(h.Len) + " to pointer to array with length " + itoa.Itoa(n))
+	if n > v.Len() {
+		panic("reflect: cannot convert slice with length " + itoa.Itoa(v.Len()) + " to pointer to array with length " + itoa.Itoa(n))
 	}
+	h := (*unsafeheader.Slice)(v.ptr)
 	return Value{t.common(), h.Data, v.flag&^(flagIndir|flagAddr|flagKindMask) | flag(Ptr)}
 }