scsi/esp-pci: add g_assert() for fix clang analyzer warning in esp_pci_io_write()

Clang static code analyzer show warning:
hw/scsi/esp-pci.c:198:9: warning: Value stored to 'size' is never read
size = 4;
^ ~

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20200325025919.21316-2-kuhn.chenqun@huawei.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>