BUG/MEDIUM: log: Don't call sample_fetch_as_type if we don't have a stream.

BUG/MEDIUM: log: Don't call sample_fetch_as_type if we don't have a stream.

In sess_build_logline(), don't attempt to call sample_fetch_as_type()
if we don't have a stream.

It used never to happen in the past before commit 09bb27c ("MEDIUM: log:
make sess_build_logline() support being called with no stream"). But now
it can happen when sess_log() is called from the lower layers (i.e. the
H2 mux got garbage when it was expecting a preface frame), and it reveals
that some sample fetch functions and some converter fnuctions which rely
on the stream don't test for its existence. For the sample fetch functions,
a durable solution is easy and would consist in adapting sample_process()
to verify the SMP_USE_* bits when the stream is not set. But for the
converters we don't have this option as they don't declare whether or not
they use a stream (which possibly is the real issue).

Thus for now let's disable sample_fetch_as_type() if a stream does not
exist, until it can be more accurately refined later.