### Godot version

v4.0.2.stable.official [7a0977ce2]

### System information

Windows 10, not platform specific

### Issue description

Methods `get_character_line` and `get_character_paragraph` return wrong indices for characters that are first in the respective line/paragraph. See the attached screenshot.

![image](https://user-images.githubusercontent.com/892180/236546781-692603be-eb09-419e-822e-d4152a66d9b6.png)


### Steps to reproduce

Run attached project.

The script goes through all characters in `RichTextLabel` and prints the respective character, line index, and paragraph index. None of them should be -1 since we are iterating through valid indices.

### Minimal reproduction project

[RichTextLabel.zip](https://github.com/godotengine/godot/files/11409399/RichTextLabel.zip)

return selection.enabled;
}
bool RichTextLabel::is_deselect_on_focus_loss_enabled() const {
	return deselect_on_focus_loss_enabled;
}
int RichTextLabel::get_selection_from() const {
	if (!selection.active || !selection.enabled) {
		return -1;
	}
	return selection.from_frame->lines[selection.from_line].char_offset + selection.from_char;
}
int RichTextLabel::get_selection_to() const {
	if (!selection.active || !selection.enabled) {
		return -1;
	}
	return selection.to_frame->lines[selection.to_line].char_offset + selection.to_char - 1;
}
void RichTextLabel::set_text(const String &p_bbcode) {