Fix new go 1.16 vet warning

See golang/go#40701 and
alecthomas/unsafeslice#4 .

Feature-Start: go-1.16

gf2p16/slice_amd64.go

@@ -63,10 +63,15 @@ func mulAndAddByteSliceLE(c T, in, out []byte, useSSSE3 bool) {
 }
 
 func castTToByteSlice(ts []T) []byte {
-	h := *(*reflect.SliceHeader)(unsafe.Pointer(&ts))
-	h.Len *= 2
-	h.Cap *= 2
-	return *(*[]byte)(unsafe.Pointer(&h))
+	tsHdr := (*reflect.SliceHeader)(unsafe.Pointer(&ts))
+	p := unsafe.Pointer(tsHdr.Data)
+
+	var bs []byte
+	bsHdr := (*reflect.SliceHeader)(unsafe.Pointer(&bs))
+	bsHdr.Data = uintptr(p)
+	bsHdr.Len = 2 * tsHdr.Len
+	bsHdr.Cap = 2 * tsHdr.Cap
+	return bs
 }
 
 func mulSlice(c T, in, out []T) {

gf2p16/slice_unsafe.go

@@ -6,10 +6,15 @@ import (
 )
 
 func castByteToTSlice(bs []byte) []T {
-	h := *(*reflect.SliceHeader)(unsafe.Pointer(&bs))
-	h.Len /= 2
-	h.Cap /= 2
-	return *(*[]T)(unsafe.Pointer(&h))
+	bsHdr := (*reflect.SliceHeader)(unsafe.Pointer(&bs))
+	p := unsafe.Pointer(bsHdr.Data)
+
+	var ts []T
+	tsHdr := (*reflect.SliceHeader)(unsafe.Pointer(&ts))
+	tsHdr.Data = uintptr(p)
+	tsHdr.Len = bsHdr.Len / 2
+	tsHdr.Cap = bsHdr.Cap / 2
+	return ts
 }
 
 func mulByteSliceLEPlatformLE(c T, in, out []byte) {