common: assert that they don't call setup_tmpctx() twice.

Otherwise, leaks will occur.  And fix up dualopend and devtool/route,
which do this!

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

devtools/route.c

@@ -28,9 +28,6 @@ static struct route_hop *least_cost(struct gossmap *map,
 	struct route_hop *path;
 	struct timemono tstart, tstop;
 
-	setup_locale();
-	setup_tmpctx();
-
 	tstart = time_mono();
 	dij = dijkstra(tmpctx, map, dst,
 		       sent, riskfactor, route_can_carry,