renepay: bugfix situation with htlcmax=htlcmin

In some weird situations it may happen that some channel along the route
could have htlcmax=htlcmin, so that the supremum of htlcmin and the
infimum of htlcmax are the same number. In that case there is only one
allowed amount that can go through that route.
Without this patch renepay would not handle correctly this cornercase.

plugins/renepay/mcf.c

@@ -1230,6 +1230,8 @@ struct list_data
 
 static inline uint64_t pseudorand_interval(uint64_t a, uint64_t b)
 {
+	if (a == b)
+		return b;
 	assert(b > a);
 	return a + pseudorand(b - a);
 }