WM: ensure wmEvent::utf8_buf isn't treated as a null terminated string

In debug mode, fill remaining bytes so functionality that relies
on null termination will alert developers to incorrect use early on.

source/blender/windowmanager/intern/wm_event_system.cc

@@ -5694,12 +5694,24 @@ void wm_event_add_ghostevent(wmWindowManager *wm, wmWindow *win, const int type,
                      utf8_buf_len);
         }
 
-        if (BLI_str_utf8_size_or_error(event.utf8_buf) == -1) {
+        const int utf8_buf_len = BLI_str_utf8_size_or_error(event.utf8_buf);
+        if (utf8_buf_len == -1) {
           CLOG_ERROR(WM_LOG_EVENTS,
                      "ghost detected an invalid unicode character '%d'",
                      int(uchar(event.utf8_buf[0])));
           event.utf8_buf[0] = '\0';
         }
+        else {
+#ifndef NDEBUG
+          /* Ensure callers don't accidentally treat this as a "string",
+           * it's not null terminated. */
+          if (utf8_buf_len > 0) {
+            for (int i = utf8_buf_len; i < ARRAY_SIZE(event.utf8_buf); i++) {
+              event.utf8_buf[i] = 0xff;
+            }
+          }
+#endif /* !NDEBUG */
+        }
       }
 
       /* NOTE(@ideasman42): Setting the modifier state based on press/release