### 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)

if (selection.active && selection.from_frame && selection.from_line >= 0 && selection.from_line < (int)selection.from_frame->lines.size()) {
		
		float line_offset = selection.from_frame->lines[selection.from_line].offset.y;
		
		for (int i = 0; i < selection.from_frame->lines[selection.from_line].text_buf->get_line_count(); i++) {
			Vector2i range = selection.from_frame->lines[selection.from_line].text_buf->get_line_range(i);
			if (range.x <= selection.from_char && range.y >= selection.from_char) {
				break;
			}
			line_offset += selection.from_frame->lines[selection.from_line].text_buf->get_line_size(i).y + theme_cache.line_separation;
		}
		
		ItemFrame *it = selection.from_frame;
		while (it->parent_frame != nullptr) {
			line_offset += it->parent_frame->lines[it->line].offset.y;
			it = it->parent_frame;
		}
		vscroll->set_value(line_offset);
	}
}
void RichTextLabel::scroll_to_paragraph(int p_paragraph) {