fix(levm): prevent inefficient memory allocation in extcodecopy (#1258)

**Motivation**

Fixes an memory inefficiency found by
[FuzzingLabs](https://github.com/FuzzingLabs) in extcodecopy opcode
implementation.

**Description**

Before, there was memory allocation even if the size to copy was zero.
Now, if that happens, returns.

Closes #1245

crates/vm/levm/src/opcode_handlers/environment.rs

@@ -314,6 +314,10 @@ impl VM {
 
         self.increase_consumed_gas(current_call_frame, gas_cost)?;
 
+        if size == 0 {
+            return Ok(OpcodeSuccess::Continue);
+        }
+
         if !is_cached {
             self.cache_from_db(&address);
         };

crates/vm/levm/tests/edge_case_tests.rs

@@ -124,3 +124,11 @@ fn test_non_compliance_returndatacopy() {
     let txreport = vm.execute(&mut current_call_frame);
     assert_eq!(txreport.result, TxResult::Revert(VMError::VeryLargeNumber));
 }
+
+#[test]
+fn test_non_compliance_extcodecopy() {
+    let mut vm = new_vm_with_bytecode(Bytes::copy_from_slice(&[88, 88, 88, 89, 60, 89])).unwrap();
+    let mut current_call_frame = vm.call_frames.pop().unwrap();
+    vm.execute(&mut current_call_frame);
+    assert_eq!(current_call_frame.stack.stack.pop().unwrap(), U256::zero());
+}