Remove runtime malloc's from SplineDrivetrain

This lets drivetrain_lib_test pass with die_on_malloc enabled.

Change-Id: If85b2ee5635fd82ba4aaf2ef18b4736cb6099d27
Signed-off-by: James Kuszmaul <jabukuszmaul+collab@gmail.com>
diff --git a/frc971/control_loops/drivetrain/drivetrain.h b/frc971/control_loops/drivetrain/drivetrain.h
index a06b965..0d100ed 100644
--- a/frc971/control_loops/drivetrain/drivetrain.h
+++ b/frc971/control_loops/drivetrain/drivetrain.h
@@ -146,9 +146,10 @@
 class DrivetrainLoop
     : public frc971::controls::ControlLoop<Goal, Position, Status, Output> {
  public:
-  // Note that we only actually store N - 1 splines, since we need to keep one
-  // fetcher free to check whether there are any new splines.
-  static constexpr size_t kNumSplineFetchers = 5;
+  // Note that we only actually store N - 1 splines consistently, since we need
+  // to keep one fetcher free to check whether there are any new splines.
+  static constexpr size_t kNumSplineFetchers =
+      SplineDrivetrain::kMaxTrajectories;
 
   // Constructs a control loop which can take a Drivetrain or defaults to the
   // drivetrain at frc971::control_loops::drivetrain