Add check in computeIsAsciiForInputs (#7032)

Summary:
Pull Request resolved: #7032

For some reason the check seems to silence the CircleCI error!
there is several seg faults in fuzzer where debugging showed that vector is null!
#7009

The problem could be a generic memory corruption, hope that this check might
make it more verbose.

Reviewed By: kgpai

Differential Revision: D50245829

fbshipit-source-id: b4188ec88531849d0f8ab4a049b717ceb122bc5a

velox/expression/Expr.cpp

@@ -1259,6 +1259,8 @@ void computeIsAsciiForInputs(
         inputValues[index]->type()->kind() == TypeKind::VARCHAR) {
       auto* vector =
           inputValues[index]->template as<SimpleVector<StringView>>();
+
+      VELOX_CHECK(vector);
       vector->computeAndSetIsAscii(rows);
     }
   }