Fix a bug in CompressedRowSparseMatrix::AppendRows

The test for CompressedRowSparseMatrix::AppendRows tries to add
a matrix of size zero, which results in an invalid pointer deferencing
even though that pointer is never written to.

Change-Id: I97dba37082bd5dad242ae1af0447a9178cd92027

docs/source/version_history.rst

diff --git a/docs/source/version_history.rst b/docs/source/version_history.rst
index 664826c..87ad50e 100644
--- a/docs/source/version_history.rst
+++ b/docs/source/version_history.rst
@@ -27,6 +27,9 @@
 
 Bug Fixes & Minor Changes
 -------------------------
+#. Fix invalid memory access bug in
+   ``CompressedRowSparseMatrix::AppendRows`` when it was called with a
+   matrix of size zero.
 #. Build position independent code when compiling Ceres statically
    (Alexander Alekhin).
 #. Fix a bug in DetectStructure (Johannes Schonberger).

internal/ceres/compressed_row_sparse_matrix.cc

diff --git a/internal/ceres/compressed_row_sparse_matrix.cc b/internal/ceres/compressed_row_sparse_matrix.cc
index 36f87a5..91d18bb 100644
--- a/internal/ceres/compressed_row_sparse_matrix.cc
+++ b/internal/ceres/compressed_row_sparse_matrix.cc
@@ -241,16 +241,24 @@
       << "The matrix being appended has: " << m.row_blocks().size()
       << " row blocks.";
 
+  if (m.num_rows() == 0) {
+    return;
+  }
+
   if (cols_.size() < num_nonzeros() + m.num_nonzeros()) {
     cols_.resize(num_nonzeros() + m.num_nonzeros());
     values_.resize(num_nonzeros() + m.num_nonzeros());
   }
 
   // Copy the contents of m into this matrix.
-  std::copy(m.cols(), m.cols() + m.num_nonzeros(), &cols_[num_nonzeros()]);
-  std::copy(m.values(),
-            m.values() + m.num_nonzeros(),
-            &values_[num_nonzeros()]);
+  DCHECK_LT(num_nonzeros(), cols_.size());
+  if (m.num_nonzeros() > 0) {
+    std::copy(m.cols(), m.cols() + m.num_nonzeros(), &cols_[num_nonzeros()]);
+    std::copy(m.values(),
+              m.values() + m.num_nonzeros(),
+              &values_[num_nonzeros()]);
+  }
+
   rows_.resize(num_rows_ + m.num_rows() + 1);
   // new_rows = [rows_, m.row() + rows_[num_rows_]]
   std::fill(rows_.begin() + num_rows_,