tls-socket: Don't fail reading if sending data failed

If data is processed that eventually includes a TLS close notify, build()
will fail after a close notify has been sent in turn.  However, propagating
that error immediately when reading prevented ever returning the data
already processed before the close notify was received.

src/libtls/tls_socket.c

@@ -188,7 +188,11 @@ static bool exchange(private_tls_socket_t *this, bool wr, bool block)
 				case SUCCESS:
 					return TRUE;
 				default:
-					return FALSE;
+					if (wr)
+					{
+						return FALSE;
+					}
+					break;
 			}
 			break;
 		}