Fix #1974: assertion when reserving an empty sparse matrix

Eigen/src/SparseCore/SparseMatrix.h

diff --git a/Eigen/src/SparseCore/SparseMatrix.h b/Eigen/src/SparseCore/SparseMatrix.h
index 2218b22..4e639d1 100644
--- a/Eigen/src/SparseCore/SparseMatrix.h
+++ b/Eigen/src/SparseCore/SparseMatrix.h
@@ -329,7 +329,8 @@
           m_outerIndex[j] = newOuterIndex[j];
           m_innerNonZeros[j] = innerNNZ;
         }
-        m_outerIndex[m_outerSize] = m_outerIndex[m_outerSize-1] + m_innerNonZeros[m_outerSize-1] + reserveSizes[m_outerSize-1];
+        if(m_outerSize>0)
+          m_outerIndex[m_outerSize] = m_outerIndex[m_outerSize-1] + m_innerNonZeros[m_outerSize-1] + reserveSizes[m_outerSize-1];
         
         m_data.resize(m_outerIndex[m_outerSize]);
       }

test/sparse_basic.cpp

diff --git a/test/sparse_basic.cpp b/test/sparse_basic.cpp
index 9e735b3..5f87621 100644
--- a/test/sparse_basic.cpp
+++ b/test/sparse_basic.cpp
@@ -662,6 +662,14 @@
     iters[0] = IteratorType(m2,0);
     iters[1] = IteratorType(m2,m2.outerSize()-1);
   }
+
+  // test reserve with empty rows/columns
+  {
+    SparseMatrixType m1(0,cols);
+    m1.reserve(ArrayXi::Constant(m1.outerSize(),1));
+    SparseMatrixType m2(rows,0);
+    m2.reserve(ArrayXi::Constant(m2.outerSize(),1));
+  }
 }