Use VTCP_Assert() also in VTCP_close()

Consistently use VTCP_Assert when asserting on the result of VTCP_Check().

lib/libvarnish/vtcp.c

@@ -342,7 +342,7 @@ VTCP_close(int *s)
 
 	i = close(*s);
 
-	assert(VTCP_Check(i));
+	VTCP_Assert(i);
 	*s = -1;
 }