evm: fix premature position truncation of byte op

nimbus/evm/interpreter/op_handlers/oph_arithmetic.nim

@@ -230,11 +230,11 @@ const
   byteOp: Vm2OpFn = proc(k: var Vm2Ctx) =
     ## 0x20, Retrieve single byte from word.
     let (position, value) = k.cpt.stack.popInt(2)    
-    let pos = position.truncate(int)
     k.cpt.stack.push:
-      if pos >= 32 or pos < 0:
+      if position >= 32.u256:
         zero(UInt256)
       else:
+        let pos = position.truncate(int)
         when system.cpuEndian == bigEndian:
           cast[array[32, byte]](value)[pos].u256
         else:

tests/test_op_arith.nim

@@ -439,5 +439,21 @@ proc opArithMain*() =
         SIGNEXTEND
       stack: "0x000000000000000000000000000000003f9b347132d29b62d161117bca8c7307"
 
+    assembler:
+      title: "BYTE with overflow pos 1"
+      code:
+        PUSH32 "0x77676767676760000000000000001002e000000000000040000000e000000000"
+        PUSH32 "0x0000000000000000000000000000000000000000000000010000000000000000"
+        BYTE
+      stack: "0x0000000000000000000000000000000000000000000000000000000000000000"
+
+    assembler:
+      title: "BYTE with overflow pos 2"
+      code:
+        PUSH32 "0x001f000000000000000000000000000000200000000100000000000000000000"
+        PUSH32 "0x0000000000000000000000000000000080000000000000000000000000000001"
+        BYTE
+      stack: "0x0000000000000000000000000000000000000000000000000000000000000000"
+
 when isMainModule:
   opArithMain()