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As found by @mongozmaki in https://github.com/esp8266/Arduino/pull/6035

With SSO implementation in String, StreamString::write generates wrong
strings under some circumstances.  Reason is that String::len() returns
strlen(sso_buf) if SSO=true but with newly written data
(in StreamString::write) the null-termination missing at the time len()
is called.

Furthermore, len() is called twice which is inefficient if SSO=true.