fixup! byte_extract lowering: also use a lower bound

Author: tautschnig

src/solvers/lowering/byte_operators.cpp

@@ -21,7 +21,7 @@ Author: Daniel Kroening, [email protected]
 static exprt unpack_rec(
   const exprt &src,
   bool little_endian,
-  const exprt &offset,
+  const exprt &offset_bytes,
   const exprt &max_bytes,
   const namespacet &ns,
   bool unpack_byte_array = false);