### 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 (!cd->subdirs.has(ds[j])) {
					PackedDir *pd = memnew(PackedDir);
					pd->name = ds[j];
					pd->parent = cd;
					cd->subdirs[pd->name] = pd;
					cd = pd;
				} else {
					cd = cd->subdirs[ds[j]];
				}
			}
		}
		String filename = simplified_path.get_file();
		
		if (!filename.is_empty()) {
			cd->files.insert(filename);
		}
	}
}
void PackedData::add_pack_source(PackSource *p_source) {
	if (p_source != nullptr) {
		sources.push_back(p_source);
	}
}
PackedData *PackedData::singleton = nullptr;
PackedData::PackedData() {
	singleton = this;
	root = memnew(PackedDir);
	add_pack_source(memnew(PackedSourcePCK));
}
void PackedData::_free_packed_dirs(PackedDir *p_dir) {