paymod: Fix the adaptive splitter partitioning

We were using the current constraints, including any shadow route and other
modifications, when computing the remainder that the second child should
use. Instead we should use the `start_constraints` on the parent payment,
which is a copy of `constraints` created in `payment_start` exactly for this
purpose.

Also added an assert for the invariant on the multiplier.

plugins/libplugin-pay.c

@@ -2335,6 +2335,8 @@ static void adaptive_splitter_cb(struct presplit_mod_data *d, struct payment *p)
 			double rand = pseudorand_double() * 0.2 + 0.9;
 			u64 mid = p->amount.millisatoshis / 2 * rand; /* Raw: multiplication */
 			bool ok;
+			/* Use the start constraints, not the ones updated by routes and shadow-routes. */
+			struct payment_constraints *pconstraints = p->start_constraints;
 
 			a = payment_new(p, NULL, p, p->modifiers);
 			b = payment_new(p, NULL, p, p->modifiers);
@@ -2342,11 +2344,15 @@ static void adaptive_splitter_cb(struct presplit_mod_data *d, struct payment *p)
 			a->amount.millisatoshis = mid;  /* Raw: split. */
 			b->amount.millisatoshis -= mid; /* Raw: split. */
 
+			double multiplier = (double)a->amount.millisatoshis / (double)p->amount.millisatoshis; /* Raw: msat division */
+			assert(multiplier >= 0.4 && multiplier < 0.6);
+
 			/* Adjust constraints since we don't want to double our
 			 * fee allowance when we split. */
-			a->constraints.fee_budget.millisatoshis *= (double)a->amount.millisatoshis / (double)p->amount.millisatoshis; /* Raw: msat division. */
+			a->constraints.fee_budget.millisatoshis = pconstraints->fee_budget.millisatoshis * multiplier; /* Raw: msat multiplication. */
+
 			ok = amount_msat_sub(&b->constraints.fee_budget,
-					     p->constraints.fee_budget,
+					     pconstraints->fee_budget,
 					     a->constraints.fee_budget);
 
 			/* Should not fail, mid is less than 55% of original