Fix crash for models without state variables

Previously, `nx_solver=0` would have in a crash when calling `amici::SUNMatrixWrapper::capacity` on 0-sized `SUNMatrixWrapper`.

Unrelated: Replace some unnecessary `std::vector::at` calls.

Author: dweindl

src/model.cpp

@@ -2875,25 +2875,26 @@ void Model::fdwdp(realtype const t, realtype const* x, bool include_static) {
 }
 
 void Model::fdwdx(realtype const t, realtype const* x, bool include_static) {
-    if (!nw)
+    if (!nw || !nx_solver)
         return;
 
     fw(t, x, include_static);
 
     derived_state_.dwdx_.zero();
     if (pythonGenerated) {
-        if (!derived_state_.dwdx_hierarchical_.at(0).capacity())
+        auto dwdx_hierarchical_0 = derived_state_.dwdx_hierarchical_.at(0);
+        if (!dwdx_hierarchical_0.capacity())
             return;
 
         fdwdw(t, x, include_static);
 
         if (include_static) {
-            derived_state_.dwdx_hierarchical_.at(0).zero();
-            fdwdx_colptrs(derived_state_.dwdx_hierarchical_.at(0));
-            fdwdx_rowvals(derived_state_.dwdx_hierarchical_.at(0));
+            dwdx_hierarchical_0.zero();
+            fdwdx_colptrs(dwdx_hierarchical_0);
+            fdwdx_rowvals(dwdx_hierarchical_0);
         }
         fdwdx(
-            derived_state_.dwdx_hierarchical_.at(0).data(), t, x,
+            dwdx_hierarchical_0.data(), t, x,
             state_.unscaledParameters.data(), state_.fixedParameters.data(),
             state_.h.data(), derived_state_.w_.data(), state_.total_cl.data(),
             derived_state_.spl_.data(), include_static