### 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)

_validate_line_caches();
	int para_count = 0;
	int to_line = main->first_invalid_line.load();
	for (int i = 0; i < to_line; i++) {
		if (main->lines[i].char_offset < p_char && p_char <= main->lines[i].char_offset + main->lines[i].char_count) {
			return para_count;
		} else {
			para_count++;
		}
	}
	return -1;
}
int RichTextLabel::get_total_character_count() const {
	// 
	int tc = 0;
	Item *it = main;
	while (it) {
		if (it->type == ITEM_TEXT) {
			ItemText *t = static_cast<ItemText *>(it);
			tc += t->text.length();
		} else if (it->type == ITEM_NEWLINE) {
			tc++;
		} else if (it->type == ITEM_IMAGE) {
			tc++;
		}
		it = _get_next_item(it, true);
	}
	return tc;
}
int RichTextLabel::get_total_glyph_count() const {