journal/audit: do not use strjoina() for arbitrary length of received string

Otherwise, when a long message is received, the assertion in strjoina()
may be triggered.

Fixes oss-fuzz#476768320 (https://issues.oss-fuzz.com/issues/476768320).
Fixes #40388.

test/fuzz/.gitattributes

@@ -1,9 +1,10 @@
-/*/*                -whitespace
-/fuzz-bus-match/*   binary
-/fuzz-compress/*    binary
-/fuzz-dhcp*/*       binary
-/fuzz-dns-packet/*  binary
-/fuzz-fido-id-desc/ binary
-/fuzz-lldp-rx/*     binary
-/fuzz-ndisc-rs/*    binary
-/*/*                generated
+/*/*                   -whitespace
+/fuzz-bus-match/*      binary
+/fuzz-compress/*       binary
+/fuzz-dhcp*/*          binary
+/fuzz-dns-packet/*     binary
+/fuzz-fido-id-desc/*   binary
+/fuzz-journald-audit/* binary
+/fuzz-lldp-rx/*        binary
+/fuzz-ndisc-rs/*       binary
+/*/*                   generated