tests: Fix BenchmarkDecode

After 2c359fc1 (decoder: Remember protocol version as last seen in a
PROTO opcode) BenchmarkDecode started to fail with

    --- FAIL: BenchmarkDecode
        ogorek_test.go:977: invalid pickle version

That happens because some input pickles now come with `PROTO 0xff`
prefix and version 0xff is indeed invalid.

-> Fix it by adjusting that prefix to be `PROTO 3` during benchmark as 3
is the most-widely used protocol version used by both py3 and py2 (via
zodbpickle).

ogorek_test.go

@@ -967,8 +967,13 @@ func BenchmarkDecode(b *testing.B) {
 				continue
 			}
 			// not prepending `PROTO <ver>` - decoder should be
-			// able to decode without it.
-			input = append(input, pickle.data...)
+			// able to decode without it. But if the pickle already
+			// comes with `PROTO 0xff` change it to `PROTO 3`.
+			data := pickle.data
+			if strings.HasPrefix(data, protoPrefixTemplate) {
+				data = string([]byte{opProto, 3}) + data[len(protoPrefixTemplate):]
+			}
+			input = append(input, data...)
 			npickle++
 		}
 	}