Return error instead of asserting in handleCompareFunction. (#3664)

The assumption is not actually true since a DropAll can happen after the
first index check.

worker/task.go

@@ -1036,8 +1036,10 @@ func (qs *queryState) handleCompareFunction(ctx context.Context, arg funcArgs) e
 	attr := arg.q.Attr
 	span.Annotatef(nil, "Attr: %s. Fname: %s", attr, arg.srcFn.fname)
 	tokenizer, err := pickTokenizer(attr, arg.srcFn.fname)
-	// We should already have checked this in getInequalityTokens.
-	x.Check(err)
+	if err != nil {
+		return err
+	}
+
 	// Only if the tokenizer that we used IsLossy, then we need to fetch
 	// and compare the actual values.
 	span.Annotatef(nil, "Tokenizer: %s, Lossy: %t", tokenizer.Name(), tokenizer.IsLossy())