Wrong behavior in marshalling invalid UTF-8 byte with EscapeHTML=false

[kz-sher]

Background:

• When EscapeHTML=true, marshalling invalid UTF-8 byte will yield \ufffd, which is correct.
• When EscapeHTML=false, marshalling invalid UTF-8 byte will keep the original byte remaining the same, which is different from encoding/json.

Findings:

• After reading source code, I found that the condition here (https://github.com/json-iterator/go/blob/v1.1.12/stream_str.go#L318) seems incorrect.
• When meeting invalid UTF-8 byte in fast path, we should break the loop and go to slow path instead.
• Also, writeStringSlowPathWithHTMLEscaped and writeStringSlowPath are better to be combined and synced with https://go.dev/src/encoding/json/encode.go#L1029

Sample:

• https://go.dev/play/p/uIKmsZQG18-