Fix a bug in covariance_impl.cc

A matrix reference was being incorrectly sized. It was using
the block_size number of rows, where as there are only block_local_size
number of rows that are present in the underlying matrix.

This did not affect the correctness of the resulting code, because
right after construction we then access a subblock with block_local_size
rows.

Thanks to William Gandler for reporting this.

Change-Id: I35e1f5a30b79e31b0b9fac79f7e361821d6073df

internal/ceres/covariance_impl.cc

diff --git a/internal/ceres/covariance_impl.cc b/internal/ceres/covariance_impl.cc
index 65fe7c6..dbfabed 100644
--- a/internal/ceres/covariance_impl.cc
+++ b/internal/ceres/covariance_impl.cc
@@ -217,7 +217,7 @@
   const int block2_local_size = block2->LocalSize();
 
   ConstMatrixRef cov(
-      covariance_matrix_->values() + rows[row_begin], block1_size, row_size);
+      covariance_matrix_->values() + rows[row_begin], block1_local_size, row_size);
 
   // Fast path when there are no local parameterizations or if the
   // user does not want it lifted to the ambient space.