Fix HDLC-FCS transmit side

If we are not able to read more packets from the user application,
the recv/read call will return 0 and set errno = EAGAIN.  In this
situation we cannot bail out but still ask the HDLC code to generate
output (sequences of flag octets).  If we fail to do that, we will
transmit all-ones pattern instaed of all-flag octets.

Change-Id: Id3bc76f1956138dcd9cb7b499f7251cd94af1329

src/intf_line.c

@@ -194,7 +194,7 @@ _e1_tx_hdlcfs(struct e1_ts *ts, uint8_t *buf, int len)
 					rv, osmo_hexdump(ts->tx_buf, rv));
 				ts->tx_len = rv; 
 				ts->tx_ofs = 0;
-			} else if (rv < 0)
+			} else if (rv < 0 && errno != EAGAIN)
 				return rv;
 		}