Correct behaviour when pushing zero-length object to DataFrame.

This triggers the warning when adding a zero-length object to a
DataFrame. It is not a valid DataFrame, so calls `warning()`, and doesn't
call `as.data.frame()`.

Author: waltersom

inst/include/Rcpp/DataFrame.h

@@ -142,7 +142,7 @@ namespace Rcpp{
                 }
             }
             for (it = Parent::begin(); it != Parent::end(); ++it) {
-                if (Rf_xlength(*it) > 1 && max_rows % Rf_xlength(*it) != 0) {
+                if (Rf_xlength(*it) == 0 || ( Rf_xlength(*it) > 1 && max_rows % Rf_xlength(*it) != 0 )) {
                     // We have a column that is not an integer fraction of the largest
                     invalid_column_size = true;
                 }

inst/tinytest/cpp/DataFrame.cpp

@@ -181,3 +181,14 @@ DataFrame DataFrame_PushReplicateLength(){
   df1.push_back(x);
   return df1;
 }
+
+// [[Rcpp::export]]
+DataFrame DataFrame_PushZeroLength(){
+  NumericVector u(2);
+  NumericVector v(0);
+
+
+  DataFrame df1 = DataFrame::create(_["u"] = u);
+  df1.push_back(v);
+  return df1;
+}

inst/tinytest/test_dataframe.R

@@ -110,3 +110,6 @@ expect_warning( DataFrame_PushWrongSize() )
 df <- data.frame( u = c(1, 0), v = c(0, 0, 0, 0), x = c(2) )
 expect_true( is.data.frame( DataFrame_PushReplicateLength() ) )
 expect_equal( DataFrame_PushReplicateLength(), df )
+
+#    test.DataFrame.PushZeroLength <- function(){
+expect_warning( DataFrame_PushZeroLength())