monitor: assert monitor_puts()'s loop invariant

Chiefly to hush up Coverity.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

monitor.c

@@ -270,6 +270,7 @@ static void monitor_puts(Monitor *mon, const char *str)
     char c;
 
     for(;;) {
+        assert(mon->outbuf_index < sizeof(mon->outbuf) - 1);
         c = *str++;
         if (c == '\0')
             break;