Change hdr_offset to signed value, so lseek64 return can be checked

The lines:

	if (instance == PRIMARY_GPT)
		hdr_offset = block_size;
	else {
		hdr_offset = lseek64(fd, 0, SEEK_END) - block_size;
	}
	if (hdr_offset < 0) {
		fprintf(stderr, "%s: Failed to get gpt header offset\n",
			__func__);
		goto error;
	}

are error checked. But previously hdr_offset could never be less than
zero because it was unsigned.

gpt-utils.cpp

@@ -442,7 +442,7 @@ static uint8_t *gpt_get_header(const char *partname, enum gpt_instance instance)
 {
 	uint8_t *hdr = NULL;
 	char devpath[PATH_MAX] = { 0 };
-	uint64_t hdr_offset = 0;
+	off_t hdr_offset = 0;
 	uint32_t block_size = 0;
 	int fd = -1;
 	if (!partname) {