invoice: fix false BROKEN log in cmp_rr_number when sort self-compares

Some qsort implementations (notably macOS) call the comparator with
the same element against itself (a == b).  cmp_rr_number assumed
rr_numbers are unique across distinct channels and logged BROKEN on
any tie, but when a == b then a->c == b->c and the tie is trivially
expected — not a data corruption.

Guard the BROKEN log with `if (a->c != b->c)`: equal channel pointers
mean the same channel entry; only different channels sharing an
rr_number is genuinely broken.  This silences the spurious BROKEN log
on macOS that caused test_xpay_limited_max_accepted_htlcs to fail with
"had BROKEN or That's weird messages".

Changelog-Fixed: invoice: no longer spuriously logs BROKEN when macOS qsort compares a routehint candidate to itself.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: nGoline

lightningd/invoice.c

@@ -606,9 +606,12 @@ static int cmp_rr_number(const struct routehint_candidate *a,
 	if (a->c->rr_number < b->c->rr_number)
 		return -1;
 
-	/* They're unique, so can't be equal */
-	log_broken(ld->log, "Two equal candidates %p and %p, channels %p and %p, rr_number %"PRIu64" and %"PRIu64,
-		   a, b, a->c, b->c, a->c->rr_number, b->c->rr_number);
+	/* rr_numbers are unique per channel; equal means same channel.
+	 * Some qsort implementations (notably macOS) compare an element
+	 * with itself, so a->c == b->c is expected and not a bug. */
+	if (a->c != b->c)
+		log_broken(ld->log, "Two equal candidates %p and %p, channels %p and %p, rr_number %"PRIu64" and %"PRIu64,
+			   a, b, a->c, b->c, a->c->rr_number, b->c->rr_number);
 	return 0;
 }