os: prevent negative array index access (#80890)

If an empty string is provided to LogMessageVerbSigSafe, the length of the
printed string is 0.

Read-only access only and the only effect it had was adding a linebreak or not.

X.Org Bug 80890 <http://bugs.freedesktop.org/show_bug.cgi?id=80890>

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

os/log.c

@@ -697,7 +697,7 @@ LogVMessageVerbSigSafe(MessageType type, int verb, const char *format, va_list a
     if (sizeof(buf) - len == 1)
         buf[len - 1] = '\n';
 
-    newline = (buf[len - 1] == '\n');
+    newline = (len > 0 && buf[len - 1] == '\n');
     LogSWrite(verb, buf, len, newline);
 }