fix off-by-one error in stage 2 prelude

stage_2/prelude.s

@@ -51,10 +51,10 @@ prelude:
   mov sp, bp
 
   mov si, [REAL_GLOBALS + STAGE_2_GPT_ENTRY_ADDR]
-  mov eax, [si + 0x20] ; Partition / boot1 start LBA lower
-  mov ebx, [si + 0x24] ; Partition / boot1 start LBA upper
+  mov eax, [si + 0x20] ; Partition / s2 start LBA lower
+  mov ebx, [si + 0x24] ; Partition / s2 start LBA upper
   mov ecx, [si + 0x28] ; Partition end LBA lower
-  mov edx, [si + 0x32] ; Partition LBA upper
+  mov edx, [si + 0x32] ; Partition end LBA upper
 
   ; Panic if the partition / boot1 starting LBA overflows 16 bits.
   or ebx, ebx
@@ -64,10 +64,15 @@ prelude:
   jnz panic_simple
   ror eax, 16
 
-  ; Calculate the s2 end LBA and panic if it overflows 16 bits.
+  ; There must be at least one sector to load.
+  mov bx, s2_bin_sectors
+  or bx, bx
+  jz panic_simple
+
+  ; Calculate the s2 end LBA (inclusive) and panic if it overflows 16 bits.
   ; n.b. ebx is zero before this so both bx and ebx can be used as the s2 end LBA.
-  mov bx, ax
-  add bx, s2_bin_sectors
+  dec bx
+  add bx, ax
   jc panic_simple
 
   ; Panic if the s2 end LBA is after the partition end LBA.