Fix offset calculation to avoid additive identity of the ring

Zero is not a part of the multiplicative group of the ring

src/sfxd.c

@@ -17,7 +17,7 @@
 #define MAX_SOURCE_DEPTH 32
 
 // Set initial offset to size to distinguish start and end
-#define FOR_OFFSET_PROBE(base, offset, index, size) for (int offset = (size) | 1, index = ((base) + offset) % (size); offset > 0; offset = (offset << 1L) % ((size) | 1), offset = offset == (size) ? (size) - 1 /* n * 2 = n - 1 (mod n + 1) */ : offset, index = ((base) + offset) % (size))
+#define FOR_OFFSET_PROBE(base, offset, index, size) for (int offset = 0, index = (base) % (size); offset >= 0; offset = ((offset + 1) << 1L) % (((size) + 1) | 1) - 1, offset = offset == (size) ? (size) - 1 /* n * 2 = n - 1 (mod n + 1) */ : offset, index = ((base) + offset) % (size))
 
 struct msg_target {
   FILE *file_handle;