out_syslog: check error after flb_log_event_decoder_next

[nokute78]

This patch is to fix error verification after flb_log_event_decoder_next.
See also #7636

No error verification except for test code.

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Enter [N/A] in the box, if an item is not applicable to your change.

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

• [N/A] 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] Run local packaging test showing all targets (including any new ones) build.
• [N/A] Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• [N/A] Documentation required for this feature

Backporting

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

Debug / Valgrind output

$ valgrind --leak-check=full bin/flb-rt-out_syslog 
==10974== Memcheck, a memory error detector
==10974== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==10974== Using Valgrind-3.18.1 and LibVEX; rerun with -h for copyright info
==10974== Command: bin/flb-rt-out_syslog
==10974== 
Test format_severity_key_rfc3164...             [ OK ]
Test format_facility_key_rfc3164...             [ OK ]
Test format_severity_facility_key_rfc3164...    [ OK ]
Test format_hostname_key_rfc3164...             [ OK ]
Test format_appname_key_rfc3164...              [ OK ]
Test format_severity_preset_rfc3164...          [ OK ]
Test format_facility_preset_rfc3164...          [ OK ]
Test format_hostname_preset_rfc3164...          [ OK ]
Test format_appname_preset_rfc3164...           [ OK ]
Test format_syslog_rfc5424...                   [ OK ]
Test format_severity_key_rfc5424...             [ OK ]
Test format_facility_key_rfc5424...             [ OK ]
Test format_severity_facility_key_rfc5424...    [ OK ]
Test format_hostname_key_rfc5424...             [ OK ]
Test format_appname_key_rfc5424...              [ OK ]
Test format_procid_key_rfc5424...               [ OK ]
Test format_msgid_key_rfc5424...                [ OK ]
Test format_sd_key_rfc5424...                   [ OK ]
Test format_severity_preset_rfc5424...          [ OK ]
Test format_facility_preset_rfc5424...          [ OK ]
Test format_hostname_preset_rfc5424...          [ OK ]
Test format_appname_preset_rfc5424...           [ OK ]
Test format_procid_preset_rfc5424...            [ OK ]
Test format_msgid_preset_rfc5424...             [ OK ]
Test allow_longer_sd_id_rfc5424...              [ OK ]
Test malformed_longer_sd_id_rfc5424...          [ OK ]
SUCCESS: All unit tests have passed.
==10974== 
==10974== HEAP SUMMARY:
==10974==     in use at exit: 0 bytes in 0 blocks
==10974==   total heap usage: 37,882 allocs, 37,882 frees, 18,071,290 bytes allocated
==10974== 
==10974== All heap blocks were freed -- no leaks are possible
==10974== 
==10974== For lists of detected and suppressed errors, rerun with: -s
==10974== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

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