follow-up prev: add testcase and minor formatting

(minor reshuffling of check so that it matches following line
 and is more clear it is a bounds check)

electrum/transaction.py

@@ -597,7 +597,6 @@ class BCDataStream(object):
 
 
 def script_GetOp(_bytes : bytes):
-    _bytes_len = len(_bytes)
     i = 0
     while i < len(_bytes):
         vch = None
@@ -618,10 +617,9 @@ def script_GetOp(_bytes : bytes):
                 try: (nSize,) = struct.unpack_from('<I', _bytes, i)
                 except struct.error: raise MalformedBitcoinScript()
                 i += 4
-
-            if nSize > _bytes_len - i:
-                raise MalformedBitcoinScript(f"Push of data element that is larger than remaining data: {nSize} vs {_bytes_len - i}")
-
+            if i + nSize > len(_bytes):
+                raise MalformedBitcoinScript(
+                    f"Push of data element that is larger than remaining data: {nSize} vs {len(_bytes) - i}")
             vch = _bytes[i:i + nSize]
             i += nSize