### 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)
int64_t pck_off = OS::get_singleton()->get_embedded_pck_offset();
		if (pck_off != 0) {
			
			for (int i = 0; i < 8; i++) {
				f->seek(pck_off);
				magic = f->get_32();
				if (magic == PACK_HEADER_MAGIC) {
#ifdef DEBUG_ENABLED
					print_verbose("PCK header found in executable pck section, loading from offset 0x" + String::num_int64(pck_off - 4, 16));
#endif
					pck_header_found = true;
					break;
				}
				pck_off++;
			}
		}
	}
	
	if (!pck_header_found) {
		
		if (p_offset != 0) {
			ERR_FAIL_V_MSG(false, "Loading self-contained executable with offset not supported.");
		}
		f->seek_end();
		f->seek(f->get_position() - 4);
		magic = f->get_32();
		if (magic == PACK_HEADER_MAGIC) {
			f->seek(f->get_position() - 12);
			uint64_t ds = f->get_64();
			f->seek(f->get_position() - ds - 8);