fix(levm): avoid overflow in calldataload (#1461)

If the offset is larger than calldata, then there is no data to show.
Return 0 to the stack

**Motivation**
The calldata operand tried to cast the value it got as an argument to
usize everytime. However, that number could be larger than usize and an
error is raised.

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**Description**
This tries to check the offset value before casting it to usize. If the
offset value (in U256) is _larger_ than the actual length of contents of
the calldata (also in U256), then we simply return 0 (since the offset
would've removed all the values from calldata).

<!-- A clear and concise general description of the changes this PR
introduces -->

<!-- Link to issues: Resolves #111, Resolves #222 -->

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

@@ -93,11 +93,19 @@ impl VM {
     ) -> Result<OpcodeSuccess, VMError> {
         self.increase_consumed_gas(current_call_frame, gas_cost::CALLDATALOAD)?;
 
-        let offset: usize = current_call_frame
-            .stack
-            .pop()?
+        let calldata_size: U256 = current_call_frame.calldata.len().into();
+
+        let offset = current_call_frame.stack.pop()?;
+
+        // If the offset is larger than the actual calldata, then you
+        // have no data to return.
+        if offset > calldata_size {
+            current_call_frame.stack.push(U256::zero())?;
+            return Ok(OpcodeSuccess::Continue);
+        };
+        let offset: usize = offset
             .try_into()
-            .map_err(|_| VMError::VeryLargeNumber)?;
+            .map_err(|_| VMError::Internal(InternalError::ConversionError))?;
 
         // All bytes after the end of the calldata are set to 0.
         let mut data = [0u8; 32];