Check return value of buf_move_to_buf for error.

If the concatenation of connection buffer and the buffer of linked
connection exceeds INT_MAX bytes, then buf_move_to_buf returns -1 as an
error value.

This value is currently casted to size_t (variable n_read) and will
erroneously lead to an increasement of variable "max_to_read".

This in turn can be used to call connection_buf_read_from_socket to
store more data inside the buffer than expected and clogging the
connection buffer.

If the linked connection buffer was able to overflow INT_MAX, the call
of buf_move_to_buf would have previously internally triggered an integer
overflow, corrupting the state of the connection buffer.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>

src/or/connection.c

@@ -3581,6 +3581,10 @@ connection_read_to_buf(connection_t *conn, ssize_t *max_to_read,
     if (conn->linked_conn) {
       result = move_buf_to_buf(conn->inbuf, conn->linked_conn->outbuf,
                                &conn->linked_conn->outbuf_flushlen);
+      if (BUG(result<0)) {
+        log_warn(LD_BUG, "reading from linked connection buffer failed.");
+        return -1;
+      }
     } else {
       result = 0;
     }