### 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)
if (pd->parent) {
				pd = pd->parent;
			}
		} else if (pd->subdirs.has(p)) {
			pd = pd->subdirs[p];
		} else {
			return nullptr;
		}
	}
	return pd;
}
Error DirAccessPack::change_dir(String p_dir) {
	PackedData::PackedDir *pd = _find_dir(p_dir);
	if (pd) {
		current = pd;
		return OK;
	} else {
		return ERR_INVALID_PARAMETER;
	}
}
String DirAccessPack::get_current_dir(bool p_include_drive) const {
	PackedData::PackedDir *pd = current;
	String p = current->name;
	while (pd->parent) {
		pd = pd->parent;
		p = pd->name.path_join(p);
	}
	return "res://" + p;
}
bool DirAccessPack::file_exists(String p_file) {