Use ParameterBlock::IsConstant in BlockJacobianWriter::CreateJacobian

Replace the check residual_block->parameter_blocks()[j]->index() != -1 with !residual_block->parameter_blocks()[j]->IsConstant() to match the loop immediately below it.

Thanks to @jwmak for reporting this.

Fixes https://github.com/ceres-solver/ceres-solver/issues/971

Change-Id: Ic2d900e04c42b3c680655a3fdfd39f7e540d2349

internal/ceres/block_jacobian_writer.cc

diff --git a/internal/ceres/block_jacobian_writer.cc b/internal/ceres/block_jacobian_writer.cc
index 34cdc1b..7e416ff 100644
--- a/internal/ceres/block_jacobian_writer.cc
+++ b/internal/ceres/block_jacobian_writer.cc
@@ -234,7 +234,7 @@
     const int num_parameter_blocks = residual_block->NumParameterBlocks();
     int num_active_parameter_blocks = 0;
     for (int j = 0; j < num_parameter_blocks; ++j) {
-      if (residual_block->parameter_blocks()[j]->index() != -1) {
+      if (!residual_block->parameter_blocks()[j]->IsConstant()) {
         num_active_parameter_blocks++;
       }
     }