Fix chd_read_header_core_file_callbacks

[xakep666]

Populate file size before calling header_read because now it performs check

	/* totalhunks is used to size the map allocation; a malformed header
	 * can otherwise request multi-GB allocations for map[] even when the
	 * file itself is tiny. Every hunk map entry consumes at least one bit
	 * in the compressed on-disk map, so totalhunks cannot legitimately
	 * exceed file_size * 8. */
	if ((uint64_t)header->totalhunks > chd->file_size * 8)
		return CHDERR_INVALID_DATA;