Throw specialized exception when bounds invalid

Inpspired by ethereum/ethereumj#1073
Cannot really use the exact test-case there as it is lgpl and we are MIT
But the input here is basically doing the same.

Author: ligi

rlp/src/main/kotlin/org/kethereum/functions/rlp/RLPDecoder.kt

@@ -14,6 +14,10 @@ private data class LengthAndOffset(val length: Int, val offset: Int)
 
 private fun ByteArray.decodeRLPWithSize(offset: Int = 0): DecodeResult {
 
+    if (offset >= size) {
+        throw IllegalRLPException("Cannot decode RLP at offset=$offset and size=$size")
+    }
+
     val value = this[offset].toInt() and 0xFF
     return when {
         value < ELEMENT_OFFSET -> DecodeResult(value.toRLP(), 1)

rlp/src/main/kotlin/org/kethereum/functions/rlp/RLPModel.kt

@@ -20,4 +20,6 @@ data class RLPElement(val bytes: ByteArray) : RLPType() {
     override fun hashCode() = Arrays.hashCode(bytes)
 }
 
-data class RLPList(val element: List<RLPType>) : RLPType()
+data class RLPList(val element: List<RLPType>) : RLPType()
+
+class IllegalRLPException(msg:String) : IllegalArgumentException(msg)

rlp/src/test/kotlin/org/kethereum/functions/rlp/TheRLPDecoder.kt

@@ -3,6 +3,7 @@ package org.kethereum.functions.rlp
 import org.assertj.core.api.Assertions.assertThat
 import org.junit.Test
 import org.walleth.khex.hexToByteArray
+import kotlin.test.assertFailsWith
 
 class TheRLPDecoder {
     @Test
@@ -13,6 +14,17 @@ class TheRLPDecoder {
         }
     }
 
+    @Test
+    fun testRLPException() {
+
+        val hex = "f912345678"
+
+        assertFailsWith<IllegalRLPException> {
+            hex.hexToByteArray().decodeRLP()
+        }
+    }
+
+
     @Test
     fun testSizeOverflow() {