Fixed integer overflow error (#7)

* Fix integer overflow error

```
 {
        // wrong command
        var stream = t.Stream.init();
        stream.add("invalid data\r\n");
        var client = TestClient.init(stream, config);
        defer client.deinit();

        try t.expectError(error.SyntaxErrorOrCommandNotFound, client.hello());
    }

```

src/reader.zig

@@ -118,11 +118,16 @@ pub const Reply = struct {
 
     // our caller made sure that data.len >= 3
     fn parse(raw: []const u8) Reply {
+        const firstByte = if (raw[0] >= '0' and raw[0] <= '9') ((@as(u16, raw[0]) - '0') * 100) else 500;
+        const secondByte = if (raw[2] >= '0' and raw[2] <= '9') ((@as(u16, raw[1]) - '0') * 10) else 0;
+        const thirdByte = if (raw[2] >= '0' and raw[2] <= '9') (raw[2] - '0') else 0;
+        const code = firstByte + secondByte + thirdByte;
+
         return .{
             .raw = raw,
             .more = raw.len > 3 and raw[3] == '-',
             .data = if (raw.len > 4) raw[4..] else "",
-            .code = ((@as(u16, raw[0]) - '0') * 100) + ((@as(u16, raw[1]) - '0') * 10) + (raw[2] - '0'),
+            .code = code,
         };
     }
 };