Preserve AX on buffered input

kernel/syscall.asm

@@ -55,6 +55,7 @@ print_string:
   jne read_buffer.chain
 
 read_buffer:
+  push ax
   push dx
   push bx
   push si
@@ -78,6 +79,7 @@ read_buffer:
   pop si
   pop bx
   pop dx
+  pop ax
   ret
 .bs:
   test bx, bx