aucoalesce: CoalesceMessages fails with "missing syscall message in compound event"

Hello,

While experimenting with this library on flatcar Linux, I noticed that the `aucoalesce.CoalesceMessages` function returns the following error when it encounters certain audit events: `missing syscall message in compound event`

If I supply the same events to the `aureport` tool (installed as a part of `auditd` package on Ubuntu), it appears to successfully parse those events - at least in the sense it does not produce error messages or exit with a non-zero status. It looks like commit 666ff1c30fe624e9fcd9a108b20fceb82331f5fa introduced the formerly-mentioned error - but it does not reference any Linux audit documentation or code that contextualizes the check.

tl;dr - I am not sure if this check is required - or if the audit logs I happened to encounter are just broken. Any kind of guidance would be appreciated :)

Below are the three audit events and the `aureport` stderr/out for them. For reference, the `--debug` argument *should* result in error messages if any events cannot be parsed and `-e` generates a report about events:

#### Failure 1

```
root@x:~# cat avc-failure-audit.log
type=AVC msg=audit(1668179838.476:649407): avc: denied { search } for pid=4059486 comm="cephcsi" name="crypto" dev="proc" ino=475090959 scontext=system_u:system_r:svirt_lxc_net_t:s0:c222,c955 tcontext=system_u:object_r:sysctl_crypto_t:s0 tclass=dir permissive=1
type=AVC msg=audit(1668179838.476:649407): avc: denied { read } for pid=4059486 comm="cephcsi" name="fips_enabled" dev="proc" ino=475090960 scontext=system_u:system_r:svirt_lxc_net_t:s0:c222,c955 tcontext=system_u:object_r:sysctl_crypto_t:s0 tclass=file permissive=1
type=AVC msg=audit(1668179838.476:649407): avc: denied { open } for pid=4059486 comm="cephcsi" path="/proc/sys/crypto/fips_enabled" dev="proc" ino=475090960 scontext=system_u:system_r:svirt_lxc_net_t:s0:c222,c955 tcontext=system_u:object_r:sysctl_crypto_t:s0 tclass=file permissive=1
root@x:~# aureport -if avc-failure-audit.log --debug -e

Event Report
===================================
# date time event type auid success
===================================
1. 11/11/2022 15:17:18 649407 AVC -2 no
root@x:~# echo $?
0
```

#### Failure 2

```
root@x:~# cat execve-uname-failure-audit.log
type=EXECVE msg=audit(1671230062.742:657491): argc=2 a0="uname" a1="-p"
type=CWD msg=audit(1671230062.742:657491): cwd="/root"
type=PATH msg=audit(1671230062.742:657491): item=0 name="/usr/bin/uname" inode=76040 dev=fe:00 mode=0100755 ouid=0 ogid=0 rdev=00:00 obj=system_u:object_r:unlabeled_t:s0 nametype=NORMAL cap_fp=0 cap_fi=0 cap_fe=0 cap_fver=0 cap_frootid=0
type=PATH msg=audit(1671230062.742:657491): item=1 name="/lib64/ld-linux-x86-64.so.2" inode=98548 dev=fe:00 mode=0100755 ouid=0 ogid=0 rdev=00:00 obj=system_u:object_r:unlabeled_t:s0 nametype=NORMAL cap_fp=0 cap_fi=0 cap_fe=0 cap_fver=0 cap_frootid=0
type=PROCTITLE msg=audit(1671230062.742:657491): proctitle=756E616D65002D70
root@x:~# aureport -if execve-uname-failure-audit.log --debug -e

Event Report
===================================
# date time event type auid success
===================================
1. 12/16/2022 22:34:22 657491 EXECVE -2 unset
root@x:~# echo $?
0
```

#### Failure 3

```
root@x:~# cat execve-ethtool-failure-audit.log
type=EXECVE msg=audit(1671230063.745:657579): argc=3 a0="/usr/sbin/ethtool" a1="-T" a2="lxc61be96845005"
type=CWD msg=audit(1671230063.745:657579): cwd="/root"
type=PATH msg=audit(1671230063.745:657579): item=0 name="/usr/sbin/ethtool" inode=162594 dev=fe:00 mode=0100755 ouid=0 ogid=0 rdev=00:00 obj=system_u:object_r:unlabeled_t:s0 nametype=NORMAL cap_fp=0 cap_fi=0 cap_fe=0 cap_fver=0 cap_frootid=0
type=PATH msg=audit(1671230063.745:657579): item=1 name="/lib64/ld-linux-x86-64.so.2" inode=98548 dev=fe:00 mode=0100755 ouid=0 ogid=0 rdev=00:00 obj=system_u:object_r:unlabeled_t:s0 nametype=NORMAL cap_fp=0 cap_fi=0 cap_fe=0 cap_fver=0 cap_frootid=0
type=PROCTITLE msg=audit(1671230063.745:657579): proctitle=2F7573722F7362696E2F657468746F6F6C002D54006C7863363162653936383435303035
root@x:~# aureport -if execve-ethtool-failure-audit.log --debug -e

Event Report
===================================
# date time event type auid success
===================================
1. 12/16/2022 22:34:23 657579 EXECVE -2 unset
root@x:~# echo $?
0
```

#### Other notes

I commented out the code that returns the error and tried running the project's Go tests with `go test ./...`. The tests ran successfully. Based on the test result and the error being generated with `errors.New` - it appears to be both untested and not programmatically checkable (e.g., `errors.As`).