BUG/MEDIUM: stream-int: don't mark as blocked an empty buffer on Rx

BUG/MEDIUM: stream-int: don't mark as blocked an empty buffer on Rx

After 8706c8131 ("BUG/MEDIUM: mux_pt: Always set CS_FL_RCV_MORE."), a
side effect caused failed receives to mark the buffer as missing room,
a flag that no other place can remove since it's empty. Ideally we need
a separate flag to mean "failed to deliver data by lack of room", but
in the mean time at the very least we must not mark as blocked an
empty buffer.

No backport is needed.