MINOR: stream-int: Notify caller when an error is reported after a rcv_buf()

For the same reason than for the commit b46784b1c ("MINOR: stream-int: Notify
caller when an error is reported after a rcv_pipe()"), we return 1 after the
call to rcv_buf() in si_cs_send() to notify the caller some processing may be
triggered.

This patch is not flagged as a bug because no strange behaviour was yet observed
without it. It is just a proactive fix to be consistent.
diff --git a/src/stream_interface.c b/src/stream_interface.c
index cf3f9d2..017d1ef 100644
--- a/src/stream_interface.c
+++ b/src/stream_interface.c
@@ -678,6 +678,9 @@
 			 * system buffers are full, we will try next time.
 			 */
 		}
+
+		if (conn->flags & CO_FL_ERROR || cs->flags & CS_FL_ERROR)
+			return 1;
 	}
  end:
 	/* We couldn't send all of our data, let the mux know we'd like to send more */