journal: do not pass a negative value to memcpy()

The message may contains only whitespaces.

Fixes #9795.

src/journal/journald-syslog.c

@@ -333,7 +333,10 @@ void server_process_syslog_message(
 
         leading_ws = strspn(buf, WHITESPACE);
 
-        if (i == raw_len)
+        if (i == 0)
+                /* The message contains only whitespaces */
+                msg = buf + raw_len;
+        else if (i == raw_len)
                 /* Nice! No need to strip anything on the end, let's optimize this a bit */
                 msg = buf + leading_ws;
         else {