### 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_FAIL_COND_MSG(f.is_null(), "File must be opened before use.");
	FileAccess::set_big_endian(p_big_endian);
	f->set_big_endian(p_big_endian);
}
Error FileAccessPack::get_error() const {
	if (eof) {
		return ERR_FILE_EOF;
	}
	return OK;
}
void FileAccessPack::flush() {
	ERR_FAIL();
}
void FileAccessPack::store_8(uint8_t p_dest) {
	ERR_FAIL();
}
void FileAccessPack::store_buffer(const uint8_t *p_src, uint64_t p_length) {
	ERR_FAIL();
}
bool FileAccessPack::file_exists(const String &p_name) {