InputText: fixed an infinite loop error happening if a custom input text callback modifies/clear BufTextLen before calling InsertChars(). (#8994, #3237)

+ misc comments.

imgui_internal.h

@@ -2484,7 +2484,7 @@ struct ImGuiContext
     ImGuiWindow*            LogWindow;
     ImFileHandle            LogFile;                            // If != NULL log to stdout/ file
     ImGuiTextBuffer         LogBuffer;                          // Accumulation buffer when log to clipboard. This is pointer so our GImGui static constructor doesn't call heap allocators.
-    const char*             LogNextPrefix;
+    const char*             LogNextPrefix;                      // See comment in LogSetNextTextDecoration(): doesn't copy underlying data, use carefully!
     const char*             LogNextSuffix;
     float                   LogLinePosY;
     bool                    LogLineFirstItem;