The tstream_pending_bytes() function is able to report how many bytes of
the incoming stream have been received but have not been consumed yet.
It returns -1 and sets 'errno' on failure.
-Otherwise it returns the number of uncomsumed bytes (it can return 0!).
+Otherwise it returns the number of unconsumed bytes (it can return 0!).
ssize_t tstream_pending_bytes(struct tstream_context *stream);