basic/ellipsize: do not assume the string is NUL-terminated when length is given

oss-fuzz flags this as:

==1==WARNING: MemorySanitizer: use-of-uninitialized-value

0. 0x7fce77519ca5 in ascii_is_valid systemd/src/basic/utf8.c:252:9
1. 0x7fce774d203c in ellipsize_mem systemd/src/basic/string-util.c:544:13
2. 0x7fce7730a299 in print_multiline systemd/src/shared/logs-show.c:244:37
3. 0x7fce772ffdf3 in output_short systemd/src/shared/logs-show.c:495:25
4. 0x7fce772f5a27 in show_journal_entry systemd/src/shared/logs-show.c:1077:15
5. 0x7fce772f66ad in show_journal systemd/src/shared/logs-show.c:1164:29
6. 0x4a2fa0 in LLVMFuzzerTestOneInput systemd/src/fuzz/fuzz-journal-remote.c:64:21
...

I didn't reproduce the issue, but this looks like an obvious error: the length
is specified, so we shouldn't use the string with any functions for normal
C-strings.

test/fuzz-regressions/meson.build

@@ -18,6 +18,7 @@ fuzz_regression_tests = '''
         fuzz-dns-packet/oss-fuzz-5465
         fuzz-journal-remote/crash-5a8f03d4c3a46fcded39527084f437e8e4b54b76
         fuzz-journal-remote/crash-96dee870ea66d03e89ac321eee28ea63a9b9aa45
+        fuzz-journal-remote/oss-fuzz-8659
         fuzz-journal-remote/oss-fuzz-8686
         fuzz-unit-file/oss-fuzz-6884
         fuzz-unit-file/oss-fuzz-6885