### 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)
for (const KeyValue<String, PackedDir *> &E : p_dir->subdirs) {
		_free_packed_dirs(E.value);
	}
	memdelete(p_dir);
}
PackedData::~PackedData() {
	for (int i = 0; i < sources.size(); i++) {
		memdelete(sources[i]);
	}
	_free_packed_dirs(root);
}
bool PackedSourcePCK::try_open_pack(const String &p_path, bool p_replace_files, uint64_t p_offset) {
	Ref<FileAccess> f = FileAccess::open(p_path, FileAccess::READ);
	if (f.is_null()) {
		return false;
	}
	bool pck_header_found = false;
	
	f->seek(p_offset);
	uint32_t magic = f->get_32();
	if (magic == PACK_HEADER_MAGIC) {
		pck_header_found = true;
	}
	
	if (!pck_header_found) {
		
		if (p_offset != 0) {
			ERR_FAIL_V_MSG(false, "Loading self-contained executable with offset not supported.");
		}