### 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)

} else if (tab_size > 0) { 
		Vector<float> tabs;
		tabs.push_back(tab_size * p_base_font->get_char_size(' ', p_base_font_size).width);
		l.text_buf->tab_align(tabs);
	}
	Item *it_to = (p_line + 1 < (int)p_frame->lines.size()) ? p_frame->lines[p_line + 1].from : nullptr;
	for (Item *it = l.from; it && it != it_to; it = _get_next_item(it)) {
		switch (it->type) {
			case ITEM_TABLE: {
				ItemTable *table = static_cast<ItemTable *>(it);
				int col_count = table->columns.size();
				for (int i = 0; i < col_count; i++) {
					table->columns[i].width = 0;
				}
				int idx = 0;
				for (Item *E : table->subitems) {
					ERR_CONTINUE(E->type != ITEM_FRAME); 
					ItemFrame *frame = static_cast<ItemFrame *>(E);
					float prev_h = 0;
					for (int i = 0; i < (int)frame->lines.size(); i++) {
						MutexLock sub_lock(frame->lines[i].text_buf->get_mutex());
						int w = _find_margin(frame->lines[i].from, p_base_font, p_base_font_size) + 1;
						prev_h = _resize_line(frame, i, p_base_font, p_base_font_size, w, prev_h);
					}
					idx++;
				}
				
				const int available_width = p_width - theme_cache.table_h_separation * (col_count - 1);
				
				int total_ratio = 0;