x86/HVM: add missing assert to stdvga's mmio_move()

... to match the IOREQ_READ path.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/hvm/stdvga.c
+++ b/xen/arch/x86/hvm/stdvga.c
@@ -519,6 +519,7 @@ static int mmio_move(struct hvm_hw_stdvg
                             put_page(dp);
                         return 0;
                     }
+                    ASSERT(!dp);
                     tmp = stdvga_mem_read(data, p->size);
                 }
                 stdvga_mem_write(addr, tmp, p->size);