Optimize decoding labels from incoming packets (#1019)

- decode is a bit faster vs str()

```
>>> ts = Timer("s.decode('utf-8', 'replace')", "s = b'TV Beneden (2)\x10_androidtvremote\x04_tcp\x05local'")
>>> ts.timeit()
0.09910525000003645
>>> ts = Timer("str(s, 'utf-8', 'replace')", "s = b'TV Beneden (2)\x10_androidtvremote\x04_tcp\x05local'")
>>> ts.timeit()
0.1304596250000145
```

zeroconf/_protocol/incoming.py

@@ -287,7 +287,7 @@ def _decode_labels_at_offset(self, off: int, labels: List[str], seen_pointers: S
 
             if length < 0x40:
                 label_idx = off + DNS_COMPRESSION_HEADER_LEN
-                labels.append(str(self.data[label_idx : label_idx + length], 'utf-8', 'replace'))
+                labels.append(self.data[label_idx : label_idx + length].decode('utf-8', 'replace'))
                 off += DNS_COMPRESSION_HEADER_LEN + length
                 continue
 
@@ -302,9 +302,9 @@ def _decode_labels_at_offset(self, off: int, labels: List[str], seen_pointers: S
                 raise IncomingDecodeError(f"DNS compression pointer at {off} points to itself")
             if link in seen_pointers:
                 raise IncomingDecodeError(f"DNS compression pointer at {off} was seen again")
-            seen_pointers.add(link)
             linked_labels = self.name_cache.get(link, [])
             if not linked_labels:
+                seen_pointers.add(link)
                 self._decode_labels_at_offset(link, linked_labels, seen_pointers)
                 self.name_cache[link] = linked_labels
             labels.extend(linked_labels)