smtp: Fixed dot stuffing being performed when no new data read

Whilst I had moved the dot stuffing code from being performed before
CRLF conversion takes place to after it, in commit 4bd860a0, I had
moved it outside the 'when something read' block of code when meant
it could perform the dot stuffing twice on partial send if nread
happened to contain the right values. It also meant the function could
potentially read past the end of buffer. This was highlighted by the
following warning:

warning: `nread' might be used uninitialized in this function