Fix segfaults in spline UI

I suspect that this actually started happening when Python upgrades
happened, since the previous logic was checking for zero-length
trajectories, but it looks like numpy may now manage zero-length arrays
differently than it did previousl.

Change-Id: Id4212d0c7ea6efcf47295841c92de3aad79072a5
diff --git a/frc971/control_loops/drivetrain/trajectory.cc b/frc971/control_loops/drivetrain/trajectory.cc
index ff60b4c..9c32483 100644
--- a/frc971/control_loops/drivetrain/trajectory.cc
+++ b/frc971/control_loops/drivetrain/trajectory.cc
@@ -735,6 +735,10 @@
 // linearization for the infal point).
 void Trajectory::CalculatePathGains() {
   const std::vector<Eigen::Matrix<double, 3, 1>> xva_plan = PlanXVA(config_.dt);
+  if (xva_plan.empty()) {
+    LOG(ERROR) << "Plan is empty--unable to plan trajectory.";
+    return;
+  }
   plan_gains_.resize(xva_plan.size());
 
   // Set up reasonable gain matrices. Current choices of gains are arbitrary