### Tested versions

4.2.stable (exported only)

### System information

Linux

### Issue description

This line is the bug:

https://github.com/godotengine/godot/blob/a311a4b162364d032b03ddf2a0e603ba40615ad7/core/io/file_access_pack.cpp#L330

`p_length` is the buffer length provided by the user, but this line does not check if the result is bigger than the length of the file.

As a result, when user tries to get a buffer that is bigger than the file (something like `file.get_buffer(99999999)`), `file.get_position()` may be bigger than `file.get_length()`.

It doesn't break Godot directly, but breaks any calculations based on `get_position()`. For example, it breaks relative seeking.

There is another variable `t_read` which is checked and should probably be used instead:

https://github.com/godotengine/godot/blob/a311a4b162364d032b03ddf2a0e603ba40615ad7/core/io/file_access_pack.cpp#L324-L328

### Steps to reproduce

Since this is a bug in `FileAccessPack`, it is only reproducible in **exported** projects (something like `FileAccessUnix` doesn't have this bug, so it can't be reproduced in the editor).

To reproduce, try to `file.get_buffer(99999999)` and then use `file.get_position()` for something.

### Minimal reproduction project (MRP)

I made a simple script that attempts to make a relative seeking and breaks because of this bug:

[FileAccessPackTest.zip](https://github.com/godotengine/godot/files/13615040/FileAccessPackTest.zip)
String nd = p_dir.replace("\\", "/");
	
	if (p_dir == "..") {
		return current->parent;
	}
	bool absolute = false;
	if (nd.begins_with("res://")) {
		nd = nd.replace_first("res://", "");
		absolute = true;
	}
	nd = nd.simplify_path();
	if (nd.is_empty()) {
		nd = ".";
	}
	if (nd.begins_with("/")) {
		nd = nd.replace_first("/", "");
		absolute = true;
	}
	Vector<String> paths = nd.split("/");
	PackedData::PackedDir *pd;
	if (absolute) {
		pd = PackedData::get_singleton()->root;
	} else {
		pd = current;
	}
	for (int i = 0; i < paths.size(); i++) {
		String p = paths[i];
		if (p == ".") {
			continue;
		} else if (p == "..") {