rpc.tlsservd.c: Check the malloc return

The NULL return is used by the following memcpy()
call.  Fail the request instead.

Unlikely to happen in practice, since the rpc.tlsservd
daemon runs as root and does not allocate a lot or memory.

Reviewed by:	rmacklem
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D60927

usr.sbin/rpc.tlsservd/rpc.tlsservd.c

@@ -358,6 +358,8 @@ rpctlssd_connect_2_svc(struct rpctlssd_connect_arg *argp,
 	assert(rqstp->rq_xprt == xprt);
 
 	ctx = malloc(sizeof(*ctx));
+	if (ctx == NULL)
+		return (FALSE);
 	memcpy(&ctx->arg, argp, sizeof(ctx->arg));
 	ctx->xid = *(uint32_t *)pthread_getspecific(xidkey);