tests/fuzzers/rlp: avoid very large input (ethereum#25109)

The oss-fuzz engine crashes due to stack overflow decoding a large nested
structure into a interface{}. This PR limits the size of the input data, so
should avoid such crashes.

tests/fuzzers/rlp/rlp_fuzzer.go

@@ -40,6 +40,9 @@ func Fuzz(input []byte) int {
 	if len(input) == 0 {
 		return 0
 	}
+	if len(input) > 500*1024 {
+		return 0
+	}
 
 	var i int
 	{