Make sure WalkResult has default values.

Since ReadingGrid::walk() may return an empty WalkResult if there are no
spans, it's important that no fields contain indeterminate values. After
this change, a local "WalkResult x;" declaration will result in all of
x's scalar fields to be 0.

src/Engine/gramambular2/reading_grid.h

@@ -163,10 +163,10 @@ class ReadingGrid {
 
   struct WalkResult {
     std::vector<NodePtr> nodes;
-    size_t totalReadings;
-    size_t vertices;
-    size_t edges;
-    uint64_t elapsedMicroseconds;
+    size_t totalReadings = 0;
+    size_t vertices = 0;
+    size_t edges = 0;
+    uint64_t elapsedMicroseconds = 0;
 
     // Convenient method for finding the node at the cursor. Returns
     // nodes.cend() if the value of cursor argument doesn't make sense. An