lib-smtp: client: Fix assertion panic occurring when DATA dot stream cannot be finish...

lib-smtp: client: Fix assertion panic occurring when DATA dot stream cannot be finished immediately.

If o_stream_finish() returns 0, o_stream_send_istream() would be called again
later which causes the panic. Instead, it should only call o_stream_finish() or
o_stream_flush() again to finish the last bit of the output.

Panic was:

Panic: file ostream.c: line 394 (o_stream_send_istream): assertion failed: (!_outstream->finished)