Cleanup: use SBuf::npos instead of npos in SBuf::append()

Cleanup: use SBuf::npos instead of npos in SBuf::append()

Small experiment. It is possible on some systems that std::npos / ::npos
is defined with a different integer size and value.
This may be what is confusing Coverity scan and producing buffer overrun
alerts in the append() c-string with default parameter case.