multiboot: bss_end_addr can be zero

The multiboot spec (https://www.gnu.org/software/grub/manual/multiboot/),
section 3.1.3, allows for bss_end_addr to be zero.

A zero bss_end_addr signifies there is no .bss section.
Suggested-by: NDaniel Kiper <daniel.kiper@oracle.com>
Signed-off-by: NJack Schwartz <jack.schwartz@oracle.com>
Reviewed-by: NDaniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: NPrasad J Pandit <pjp@fedoraproject.org>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

hw/i386/multiboot.c

@@ -233,12 +233,6 @@ int load_multiboot(FWCfgState *fw_cfg,
         mh_entry_addr = ldl_p(header+i+28);
 
         if (mh_load_end_addr) {
-            if (mh_bss_end_addr < mh_load_addr) {
-                fprintf(stderr, "invalid mh_bss_end_addr address\n");
-                exit(1);
-            }
-            mb_kernel_size = mh_bss_end_addr - mh_load_addr;
-
             if (mh_load_end_addr < mh_load_addr) {
                 fprintf(stderr, "invalid mh_load_end_addr address\n");
                 exit(1);
@@ -249,8 +243,16 @@ int load_multiboot(FWCfgState *fw_cfg,
                 fprintf(stderr, "invalid kernel_file_size\n");
                 exit(1);
             }
-            mb_kernel_size = kernel_file_size - mb_kernel_text_offset;
-            mb_load_size = mb_kernel_size;
+            mb_load_size = kernel_file_size - mb_kernel_text_offset;
+        }
+        if (mh_bss_end_addr) {
+            if (mh_bss_end_addr < (mh_load_addr + mb_load_size)) {
+                fprintf(stderr, "invalid mh_bss_end_addr address\n");
+                exit(1);
+            }
+            mb_kernel_size = mh_bss_end_addr - mh_load_addr;
+        } else {
+            mb_kernel_size = mb_load_size;
         }
 
         /* Valid if mh_flags sets MULTIBOOT_HEADER_HAS_VBE.