[IA64] xen: Fix return value check in xencomm_vtop()

In case of error, the function follow_page() returns ERR_PTR()
or NULL pointer. The NULL test in the error handling should be
replaced with IS_ERR_OR_NULL().

dpatch engine is used to auto generated this patch.
(https://github.com/weiyj/dpatch

)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Tony Luck <tony.luck@intel.com>