Core dump when tls is off for output plugins which have TLS optional

[phiremande]

Bug Report

Describe the bug
When tls is set to Off and

1. tls.crt_file and/or tls.key_file is set to a file which is not present.
2. Or if the /etc/ssl/certs doesn't contain cert/key
   In these cases the fluent-bit crashes.
   An example plugin where TLS is optional is the output plugin kafka-rest.

To Reproduce
[OUTPUT]
Name kafka-rest
Match my_input
Host localhost
Port 8082
Topic my_kafka_test
Message_Key my_key

tls         Off
tls.verify  Off
tls.ca_file     /tmp/fluent-bit/certificate.pem
tls.crt_file    /tmp/fluent-bit/certificate.pem
tls.key_file    /tmp/fluent-bit/key.pem

[2019/12/25 01:10:30] [error] [TLS] Invalid Key file: /tmp/fluent-bit/key.pem
[2019/12/25 01:10:30] [error] [output kafka-rest.0] error initializing TLS context
[2019/12/25 01:10:30] [ info] [input] pausing tcp.0
[2019/12/25 01:10:30] [ info] [input] pausing tail.1
[2019/12/25 01:10:30] [ warn] [input] cannot disable event for tail.1
[2019/12/25 01:10:30] [ warn] [input] cannot disable event for tail.1
[2019/12/25 01:10:30] [ warn] [input] cannot disable event for tail.1
free(): invalid size
Aborted (core dumped)

• Steps to reproduce the problem:
  Use a configuration as provided above and run fluent-bit. Else remove certs from /etc/ssl/certs and remove the tls.crt_file and tls.key_file entries from the above configuration.

Expected behavior
When tls is "Off" irrespective of whether the key/cert is present or not fluent-bit should run. key/cert are not necessary when tls is turned off.

Your Environment

• Version used: latest from master
• Configuration:
• Environment name and version (e.g. Kubernetes? What version?):
• Server type and version:
• Operating System and version:
  Ubuntu 18.04
• Filters and plugins:
  kafka-rest output plugin.

Additional context
Trying to send logs to kafka rest proxy with and without tls.

[edsiper]

Fixed with #1841

thanks!