### 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)
ERR_PRINT("Can't open pack-referenced file.");
	return ERR_UNAVAILABLE;
}
bool FileAccessPack::is_open() const {
	if (f.is_valid()) {
		return f->is_open();
	} else {
		return false;
	}
}
void FileAccessPack::seek(uint64_t p_position) {
	ERR_FAIL_COND_MSG(f.is_null(), "File must be opened before use.");
	if (p_position > pf.size) {
		eof = true;
	} else {
		eof = false;
	}
	f->seek(off + p_position);
	pos = p_position;
}
void FileAccessPack::seek_end(int64_t p_position) {
	seek(pf.size + p_position);
}
uint64_t FileAccessPack::get_position() const {
	return pos;
}
uint64_t FileAccessPack::get_length() const {
	return pf.size;
}
bool FileAccessPack::eof_reached() const {