engine: fix segfault in flb_config_init error case

[alecholmes]

This fixes a segfault in flb_config_init that occurs when the call to flb_config_task_map_resize fails and then calls flb_config_exit, which subsequently dereferences invalid config->multiline_parsers.

This was picked up by the fuzzer in CI and can be reproduced on master with this input:

echo "DgAAAAAHBy8rLi8vLv8QLi8rLy97AT8P/////////xYHB3sBAQAgPy79JjIxNDc0ODM2NDljATg3AAA=" | base64 -d > /tmp/repro_bin
bin/flb-it-fuzz-signv4_fuzzer_OSSFUZZ /tmp/repro_bin

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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 (see below)

• 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

• Backport to latest stable release.

Test output

Before:

INFO: Running with entropic power schedule (0xFF, 100).
INFO: Seed: 151106316
bin/flb-it-fuzz-signv4_fuzzer_OSSFUZZ: Running 1 inputs 1 time(s) each.
Running: /tmp/repro_bin
[2025/05/19 15:58:53] [error] [/home/alec/dev/fluent-bit/src/flb_config.c:1030 errno=0] Success
[2025/05/19 15:58:53] [error] [config] task map resize failed
AddressSanitizer:DEADLYSIGNAL
=================================================================
==816689==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000008 (pc 0xaaaab7c13da4 bp 0xffffdca6b760 sp 0xffffdca6b730 T0)
==816689==The signal is caused by a READ memory access.
==816689==Hint: address points to the zero page.
    #0 0xaaaab7c13da4 in flb_ml_parser_destroy_all /home/alec/dev/fluent-bit/src/multiline/flb_ml_parser.c:343:5
    #1 0xaaaab7ba14dc in flb_ml_exit /home/alec/dev/fluent-bit/src/multiline/flb_ml.c:1561:5
    #2 0xaaaab7ba5c08 in flb_parser_exit /home/alec/dev/fluent-bit/src/flb_parser.c:398:5
    #3 0xaaaab7b7a974 in flb_config_exit /home/alec/dev/fluent-bit/src/flb_config.c:514:5
    #4 0xaaaab7b7a1c8 in flb_config_init /home/alec/dev/fluent-bit/src/flb_config.c:353:9
    #5 0xaaaab7b75300 in LLVMFuzzerTestOneInput /home/alec/dev/fluent-bit/tests/internal/fuzzers/signv4_fuzzer.c:60:19
    #6 0xaaaab7aaf044 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) (/home/alec/dev/fluent-bit/build/bin/flb-it-fuzz-signv4_fuzzer_OSSFUZZ+0x8f044) (BuildId: c5dec4264846745ff5ec0601ce134a381aca66c3)
    #7 0xaaaab7a9a578 in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) (/home/alec/dev/fluent-bit/build/bin/flb-it-fuzz-signv4_fuzzer_OSSFUZZ+0x7a578) (BuildId: c5dec4264846745ff5ec0601ce134a381aca66c3)
    #8 0xaaaab7a9faf4 in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) (/home/alec/dev/fluent-bit/build/bin/flb-it-fuzz-signv4_fuzzer_OSSFUZZ+0x7faf4) (BuildId: c5dec4264846745ff5ec0601ce134a381aca66c3)
    #9 0xaaaab7ac70ec in main (/home/alec/dev/fluent-bit/build/bin/flb-it-fuzz-signv4_fuzzer_OSSFUZZ+0xa70ec) (BuildId: c5dec4264846745ff5ec0601ce134a381aca66c3)
    #10 0xffff8fba73f8 in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
    #11 0xffff8fba74c8 in __libc_start_main csu/../csu/libc-start.c:392:3
    #12 0xaaaab7a9602c in _start (/home/alec/dev/fluent-bit/build/bin/flb-it-fuzz-signv4_fuzzer_OSSFUZZ+0x7602c) (BuildId: c5dec4264846745ff5ec0601ce134a381aca66c3)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /home/alec/dev/fluent-bit/src/multiline/flb_ml_parser.c:343:5 in flb_ml_parser_destroy_all
==816689==ABORTING

After:

INFO: Running with entropic power schedule (0xFF, 100).
INFO: Seed: 277839252
bin/flb-it-fuzz-signv4_fuzzer_OSSFUZZ: Running 1 inputs 1 time(s) each.
Running: /tmp/repro_bin
[2025/05/19 15:57:49] [error] [/home/alec/dev/fluent-bit/src/flb_config.c:1030 errno=0] Success
[2025/05/19 15:57:49] [error] [config] task map resize failed
[2025/05/19 15:57:49] [error] [/home/alec/dev/fluent-bit/src/flb_config.c:1030 errno=9] Bad file descriptor
[2025/05/19 15:57:49] [error] [config] task map resize failed
Executed /tmp/repro_bin in 2 ms

[edsiper]

thanks for this contribution.

note: CI was complaining about the PR subject format, it needs to be similar to the git commits, already fixed.