fuzz-systemctl: adjust size limit

Follow-up for 719b7d4dc2.

The size of the current reproducer is 250KB. Hence, 16KB should be
enough, but still we can test most arguments within the size.

Hopefully fixes oss-fuzz#53552.

src/systemctl/fuzz-systemctl-parse-argv.c

@@ -19,7 +19,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         _cleanup_close_ int orig_stdout_fd = -1;
         int r;
 
-        if (size > 4*1024*1024)
+        if (size > 16*1024)
                 return 0; /* See the comment below about the limit for strv_length(). */
 
         /* We don't want to fill the logs with messages about parse errors.