BUG/MAJOR: htx: Don't swap buffers for empty HTX message with an error

BUG/MAJOR: htx: Don't swap buffers for empty HTX message with an error

Recent fix of some HTX muxes to drain remaining data when the stream is in
closed state revealed a bug, mainly due to a corner case of the HTX API.

It is possible to have an empty HTX message with a parsing/internal
error. In that case, the underlying buffer remains full. It is mandatory to
prevent any buffer release and be sure the error will be handeled.

On the other end, at several places, when data must be transfer from an HTX
message to another one, we try to swap underlying buffers instead of
performing a bloc-per-bloc copy. To do so, we rely on b_xfer() function. One
condition is that the destination message must be empty. And here is the
issue. The HTX message can be empty but the buffer can also be full because
an HTX error was triggered earlier and not handled yet. In that case,
attempting to call b_xfer() leads to a crash because the destination buffer
is full. It is not expected to call b_xfer() if there is not enough space in
the destination buffer.

So, it appears the HTX API should be improved/fixed but first of all, the
bug must be fixed. Especially because stable versions are also affected. The
htx_is_empty_noerr() function was added to know if a HTX message is empty
and no error was reported on it. And this function is now used, instead of
htx_is_empty(), to know if we can safely swap the underlying buffers or not.

the FCGI, H2 and QUIC multiplexers are concerned. The HTTP client and the
applet API were also fixed while it seems harder to trigger the bug at these
places.

The fix must be backported to all supported versions.