HeAP: Don't call Eigen if system is empty

Fixes #259

Signed-off-by: David Shah <dave@ds0.me>

common/placer_heap.cc

@@ -100,7 +100,8 @@ template <typename T> struct EquationSystem
     void solve(std::vector<T> &x)
     {
         using namespace Eigen;
-
+        if (x.empty())
+            return;
         NPNR_ASSERT(x.size() == A.size());
 
         VectorXd vx(x.size()), vb(rhs.size());

tests

@@ -1 +1 @@
-Subproject commit 32a683071758ee59d47e2c5cb29c87882993facd
+Subproject commit fb53a97322c32620385af6657596681c0044a32e