Fix wrong allocated vector length in RowSlice

The length of the returned DVec should correspond to the
the number of elements in the slice and not the number of rows in the
matrix.

src/structs/dmat.rs

@@ -559,7 +559,7 @@ impl<N: Copy> RowSlice<DVec<N>> for DMat<N> {
         assert!(col_start < col_end);
         assert!(col_end <= self.ncols);
         let mut slice : DVec<N> = unsafe {
-            DVec::new_uninitialized(self.nrows)
+            DVec::new_uninitialized(col_end - col_start)
         };
         let mut slice_idx = 0;
         for col_id in col_start..col_end {