Fix race condition in log_println (use-after-free)

The check for logready was done BEFORE acquiring the lock, causing a race
condition with log_close(). If log_close() runs between the check and
the lock acquisition, the log file would be closed but log_println would
still try to write to it (use-after-free).

Now the logready/logp check is inside the critical section.

src/server/log.c

@@ -70,11 +70,14 @@ log_init(stbbr_log_t loglevel)
 void
 log_println(stbbr_log_t loglevel, const char * const msg, ...)
 {
-    if (!logready || loglevel < minlevel) {
+    pthread_mutex_lock(&loglock);
+    
+    // Check logready AFTER acquiring lock to prevent race with log_close
+    if (!logready || !logp || loglevel < minlevel) {
+        pthread_mutex_unlock(&loglock);
         return;
     }
 
-    pthread_mutex_lock(&loglock);
     va_list arg;
     va_start(arg, msg);
     GString *fmt_msg = g_string_new(NULL);