gossipd: fix uninitialized free on short_route in goto path

Fix a path where tal_free is called on an uninitialized variable

If the first `goto bad_total` executes, then that path has
uninitialized `short_route` but bad_total passes through to `out`
whose first call is tal_free(short_route).

This was noticed by a maybe-uninitialized heuristic on gcc 7.4.0:

gossipd/routing.c: In function ‘find_shorter_route’:
gossipd/routing.c:1096:2: error: ‘short_route’ may be used
uninitialized in this function [-Werror=maybe-uninitialized]
  tal_free(short_route);

Reported-by: @ZmnSCPxj <https://github.com/ElementsProject/lightning/pull/2674#issuecomment-495617253>
Signed-off-by: William Casarin <jb55@jb55.com>

gossipd/routing.c

@@ -988,7 +988,7 @@ find_shorter_route(const tal_t *ctx, struct routing_state *rstate,
 		   struct amount_msat *fee)
 {
 	struct unvisited *unvisited;
-	struct chan **short_route;
+	struct chan **short_route = NULL;
 	struct amount_msat long_cost, short_cost, cost_diff;
 	u64 min_bias, max_bias;
 	double riskfactor;