pc-bios/s390-ccw: don't try to read the next block if end of chunk is…

… reached

Don't read the block if a null block number is reached, because this means that
the end of chunk is reached.

Reviewed-by: Collin Walling <walling@linux.ibm.com>
Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Message-Id: <20210416074736.17409-1-mhartmay@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

pc-bios/s390-ccw/bootmap.c

@@ -213,7 +213,7 @@ static int eckd_get_boot_menu_index(block_number_t s1b_block_nr)
                 next_block_nr = eckd_block_num(&s1b->seek[i + 1].chs);
             }
 
-            if (next_block_nr) {
+            if (next_block_nr && !is_null_block_number(next_block_nr)) {
                 read_block(next_block_nr, s2_next_blk,
                            "Cannot read stage2 boot loader");
             }