* Fix#3545: XGDMatrixCreateFromCSCEx silently discards empty trailing rows
Description: The bug is triggered when
1. The data matrix has empty rows at the bottom. More precisely, the rows
`n-k+1`, `n-k+2`, ..., `n` of the matrix have missing values in all
dimensions (`n` number of instances, `k` number of trailing rows)
2. The data matrix is given as Compressed Sparse Column (CSC) format.
Diagnosis: When the CSC matrix is converted to Compressed Sparse Row (CSR)
format (this is common format used for DMatrix), the trailing empty rows
are silently ignored. More specifically, the row pointer (`offset`) of the
newly created CSR matrix does not take account of these rows.
Fix: Modify the row pointer.
* Add regression test
109473dae2