vet: fix vet warnings

spew/dump_test.go

diff --git a/spew/dump_test.go b/spew/dump_test.go
index 5aad9c7..4a31a2e 100644
--- a/spew/dump_test.go
+++ b/spew/dump_test.go
@@ -768,7 +768,7 @@
 
 func addUnsafePointerDumpTests() {
 	// Null pointer.
-	v := unsafe.Pointer(uintptr(0))
+	v := unsafe.Pointer(nil)
 	nv := (*unsafe.Pointer)(nil)
 	pv := &v
 	vAddr := fmt.Sprintf("%p", pv)

spew/format_test.go

diff --git a/spew/format_test.go b/spew/format_test.go
index 0719eb9..87ee965 100644
--- a/spew/format_test.go
+++ b/spew/format_test.go
@@ -1083,7 +1083,7 @@
 
 func addUnsafePointerFormatterTests() {
 	// Null pointer.
-	v := unsafe.Pointer(uintptr(0))
+	v := unsafe.Pointer(nil)
 	nv := (*unsafe.Pointer)(nil)
 	pv := &v
 	vAddr := fmt.Sprintf("%p", pv)