Fix -Wno-maybe-uninitialized error

One of my compilers couldn't deduce that Chunk was assigned before being
used based on the context.  Make it more obvious...

Change-Id: I1e331c6c02a2783eff8db03c6359823ac3c46686

internal/ceres/schur_eliminator.h

diff --git a/internal/ceres/schur_eliminator.h b/internal/ceres/schur_eliminator.h
index 45bb6c5..a5a4569 100644
--- a/internal/ceres/schur_eliminator.h
+++ b/internal/ceres/schur_eliminator.h
@@ -273,7 +273,7 @@
   // buffer_layout[z2] = y1 * z1 + y1 * z5
   typedef std::map<int, int> BufferLayoutType;
   struct Chunk {
-    Chunk() : size(0) {}
+    Chunk(int start) : size(0), start(start) {}
     int size;
     int start;
     BufferLayoutType buffer_layout;

internal/ceres/schur_eliminator_impl.h

diff --git a/internal/ceres/schur_eliminator_impl.h b/internal/ceres/schur_eliminator_impl.h
index 1f0b4fa..9b2ff0e 100644
--- a/internal/ceres/schur_eliminator_impl.h
+++ b/internal/ceres/schur_eliminator_impl.h
@@ -125,10 +125,8 @@
       break;
     }
 
-    chunks_.push_back(Chunk());
+    chunks_.push_back(Chunk(r));
     Chunk& chunk = chunks_.back();
-    chunk.size = 0;
-    chunk.start = r;
     int buffer_size = 0;
     const int e_block_size = bs->cols[chunk_block_id].size;