Don't test for xpt not being NULL before calling svc_xprt_free(..)

svc_xprt_alloc(..) will always return initialized memory as it uses
mem_alloc(..) under the covers, which uses malloc(.., M_WAITOK, ..).

MFC after: 1 week
Reported by: Coverity
CID: 1007341
Sponsored by: EMC / Isilon Storage Division