encoding/pem: TestFuzz fails again

Apparent repeat of #19829. Maybe I didn't fix it well enough last time.

```
--- FAIL: TestFuzz (0.00s)
	pem_test.go:242: Encode of &pem.Block{Type:"𥁵\U0005cac4\U0002ddf6\U0010ae54\U000b322e\U000f1bd4\U00073f73:\U000ec347\U0008e656\ue478\U000ee00d鰭𢢗\U000acf01\U00109e18\U000d1766\U00046126", Headers:map[string]string{}, Bytes:[]uint8{}} decoded as (*pem.Block)(nil)
FAIL
FAIL	encoding/pem	0.033s
```