Fix a panic in the fluentd log driver when the collector address uses
TLS, cannot be connected to, and fluentd-async is enabled.
A failed tls.DialWithDialer returns a nil *tls.Conn, which connect()
assigned to the net.Conn field. The field then held a non-nil interface
wrapping a nil pointer, passed the nil check in close(), and crashed
the daemon with a nil pointer dereference in tls.(*Conn).Close.
full diff: https://github.com/fluent/fluent-logger-golang/compare/v1.10.1...v1.10.2
Signed-off-by: Takumi Akasaka <takumiakasaka1231@gmail.com>