in_exec: use flb_calloc to fill NULL (#5715)

[nokute78]

#5715 (comment)
Valgrind reports using uninitialized ctx->parser_name.
I modified to fill NULL using flb_calloc.

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

• Example configuration file for the change
• Debug log output from testing the change

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

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• [N/A] Attached local packaging test output showing all targets (including any new ones) build.

Documentation

• [N/A] Documentation required for this feature

Backporting

• [N/A] Backport to latest stable release.

Configuration

[SERVICE]
    Flush        1
    Daemon       Off
    Parsers_File parsers.conf 
    
[INPUT]
    Name          tail
    Tag           json
    Path          ./static/*.json
    Path_Key       filename
    DB            ./static/json.db
    Parser         docker

[INPUT]
    Name          exec
    Tag           bin
    Command       ls ./static/*.bin
    Interval_Sec  2
    Interval_NSec 0

[OUTPUT]
    Name null
    Match *

Debug log/Valgrind output

$ valgrind --leak-check=full ../bin/fluent-bit -c a.conf 
==29660== Memcheck, a memory error detector
==29660== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==29660== Using Valgrind-3.15.0 and LibVEX; rerun with -h for copyright info
==29660== Command: ../bin/fluent-bit -c a.conf
==29660== 
Fluent Bit v2.0.0
* Copyright (C) 2015-2022 The Fluent Bit Authors
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

[2022/07/14 21:57:32] [ info] [fluent bit] version=2.0.0, commit=82d1819c65, pid=29660
[2022/07/14 21:57:32] [ info] [storage] version=1.2.0, type=memory-only, sync=normal, checksum=disabled, max_chunks_up=128
[2022/07/14 21:57:32] [ info] [cmetrics] version=0.3.5
[2022/07/14 21:57:33] [ info] [sp] stream processor started
[2022/07/14 21:57:33] [ info] [output:null:null.0] worker #0 started
[2022/07/14 21:57:33] [ info] [input:tail:tail.0] inotify_fs_add(): inode=1450535 watch_fd=1 name=./static/a.json
^C[2022/07/14 21:57:34] [engine] caught signal (SIGINT)
[2022/07/14 21:57:34] [ warn] [engine] service will shutdown in max 5 seconds
[2022/07/14 21:57:34] [ info] [input] pausing tail.0
[2022/07/14 21:57:35] [ info] [engine] service has stopped (0 pending tasks)
[2022/07/14 21:57:35] [ info] [input] pausing tail.0
[2022/07/14 21:57:35] [ info] [input:tail:tail.0] inotify_fs_remove(): inode=1450535 watch_fd=1
[2022/07/14 21:57:35] [ info] [output:null:null.0] thread worker #0 stopping...
[2022/07/14 21:57:35] [ info] [output:null:null.0] thread worker #0 stopped
==29660== 
==29660== HEAP SUMMARY:
==29660==     in use at exit: 0 bytes in 0 blocks
==29660==   total heap usage: 3,363 allocs, 3,363 frees, 1,017,442 bytes allocated
==29660== 
==29660== All heap blocks were freed -- no leaks are possible
==29660== 
==29660== For lists of detected and suppressed errors, rerun with: -s
==29660== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[nokute78]

I rebased master and CI passed.

[nokute78]

@edsiper Could you merge this ?
User reported another issue #5848 .