out_splunk: set channel len on init

[nokute78]

This patch is to fix below comment issue.
'channel' of out_splunk doesn't work correctly since channel length is not initialized.
Then it is failed to append header.
23aaf15#commitcomment-53317894

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Testing
Before we can approve your change; please submit the following in a comment:

• Example configuration file for the change
• [N/A] Debug log output from testing the change

• Attached Valgrind output that shows no leaks or memory corruption was found

Documentation

• [N/A] Documentation required for this feature

Example Configuration

[INPUT]
    Name dummy
    Samples 1

[OUTPUT]
    Name splunk
    Http_User nanashi
    Channel bd3b4270-3c26-4127-a677-8643d44f3984

Valgrind output

1. exec nc -l 8088
2. exec fluent-bit -c splunk.conf

$ nc -l 8088
POST /services/collector/event HTTP/1.1
Host: 127.0.0.1:8088
Content-Length: 54
User-Agent: Fluent-Bit
Authorization: Basic bmFuYXNoaTo=
X-Splunk-Request-Channel: bd3b4270-3c26-4127-a677-8643d44f3984

{"time":1625987478.772393,"event":{"message":"dummy"}}^C

$ valgrind bin/fluent-bit -c ~/git/WORKTREE/v1.8.1/build/splunk.conf 
==114819== Memcheck, a memory error detector
==114819== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==114819== Using Valgrind-3.15.0 and LibVEX; rerun with -h for copyright info
==114819== Command: bin/fluent-bit -c /home/taka/git/WORKTREE/v1.8.1/build/splunk.conf
==114819== 
Fluent Bit v1.9.0
* Copyright (C) 2019-2021 The Fluent Bit Authors
* Copyright (C) 2015-2018 Treasure Data
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

[2021/07/11 16:11:17] [ info] [engine] started (pid=114819)
[2021/07/11 16:11:17] [ info] [storage] version=1.1.1, initializing...
[2021/07/11 16:11:17] [ info] [storage] in-memory
[2021/07/11 16:11:17] [ info] [storage] normal synchronization mode, checksum disabled, max_chunks_up=128
[2021/07/11 16:11:17] [ info] [cmetrics] version=0.1.3
[2021/07/11 16:11:17] [ info] [sp] stream processor started
==114819== Warning: client switching stacks?  SP change: 0x57e48b8 --> 0x4c81e00
==114819==          to suppress, use: --max-stackframe=11938488 or greater
==114819== Warning: client switching stacks?  SP change: 0x4c81d78 --> 0x57e48b8
==114819==          to suppress, use: --max-stackframe=11938624 or greater
==114819== Warning: client switching stacks?  SP change: 0x57e48b8 --> 0x4c81d78
==114819==          to suppress, use: --max-stackframe=11938624 or greater
==114819==          further instances of this message will not be shown.
^C[2021/07/11 16:11:24] [engine] caught signal (SIGINT)
[2021/07/11 16:11:24] [ warn] [engine] service will stop in 5 seconds
[2021/07/11 16:11:25] [error] [http_client] broken connection to 127.0.0.1:8088 ?
[2021/07/11 16:11:25] [ warn] [output:splunk:splunk.0] http_do=-1
[2021/07/11 16:11:25] [ warn] [engine] failed to flush chunk '114819-1625987478.782892611.flb', retry in 8 seconds: task_id=0, input=dummy.0 > output=splunk.0 (out_id=0)
[2021/07/11 16:11:28] [ info] [engine] service stopped
==114819== 
==114819== HEAP SUMMARY:
==114819==     in use at exit: 0 bytes in 0 blocks
==114819==   total heap usage: 1,062 allocs, 1,062 frees, 853,462 bytes allocated
==114819== 
==114819== All heap blocks were freed -- no leaks are possible
==114819== 
==114819== For lists of detected and suppressed errors, rerun with: -s
==114819== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

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