pc-bios/s390-ccw: break loop if a null block number is reached

Break the loop if `cur_block_nr` is a null block number because this
means that the end of chunk is reached. In this case we will try to
boot the default entry.

Fixes: ba831b25 ("s390-ccw: read stage2 boot loader data to find menu")
Reviewed-by: Collin Walling <walling@linux.ibm.com>
Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Message-Id: <20200924085926.21709-3-mhartmay@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

pc-bios/s390-ccw/bootmap.c

@@ -192,7 +192,7 @@ static int eckd_get_boot_menu_index(block_number_t s1b_block_nr)
     for (i = 0; i < STAGE2_BLK_CNT_MAX; i++) {
         cur_block_nr = eckd_block_num(&s1b->seek[i].chs);
 
-        if (!cur_block_nr) {
+        if (!cur_block_nr || is_null_block_number(cur_block_nr)) {
             break;
         }