fuzz-tests: Replace hardcoded chunk size with iteration over sizes

Changelog-None: The exisiting fuzz test only extracts chunks of
a fixed size (8) from the fuzzer's input. Replace this with an
iteration over a set of chunk sizes (1 to BIGSIZE_MAX_LEN) for
better coverage.

While at it, get rid of the check `if (bs != 0)` because 0 is a
valid value for bigsize_t as well.

Author: Chandra Pratap

tests/fuzz/fuzz-bigsize.c

@@ -14,21 +14,20 @@ void run(const uint8_t *data, size_t size)
 	const uint8_t **wire_chunks, *wire_ptr;
 	size_t wire_max;
 
-	wire_chunks = get_chunks(NULL, data, size, 8);
-	for (size_t i = 0; i < tal_count(wire_chunks); i++) {
-		wire_max = tal_count(wire_chunks[i]);
-		wire_ptr = wire_chunks[i];
+	for (size_t max = 1; max <= BIGSIZE_MAX_LEN; max++) {
+		wire_chunks = get_chunks(NULL, data, size, max);
+		for (size_t i = 0; i < tal_count(wire_chunks); i++) {
+			wire_max = tal_count(wire_chunks[i]);
+			wire_ptr = wire_chunks[i];
 
-		bigsize_t bs = fromwire_bigsize(&wire_ptr, &wire_max);
-		if (bs != 0) {
-			/* We have a valid bigsize type, now we should not error. */
+			bigsize_t bs = fromwire_bigsize(&wire_ptr, &wire_max);
 			assert(bigsize_put(buff, bs) > 0);
 			assert(bigsize_len(bs));
 
 			wire_buff = tal_arr(NULL, uint8_t, 8);
 			towire_bigsize(&wire_buff, bs);
 			tal_free(wire_buff);
 		}
+		tal_free(wire_chunks);
 	}
-	tal_free(wire_chunks);
 }