Fix memory issues found by ASAN

- alloc-dealloc mismatch
- missing deallocation of features in derived class colvarbias_abf

src/colvarbias_abf.cpp

@@ -353,7 +353,10 @@ colvarbias_abf::~colvarbias_abf()
     delete_if_non_null(global_czar_gradients)
     delete_if_non_null(global_czar_pmf)
   }
-  delete_if_non_null(system_force)
+  if (system_force) delete[] system_force;
+  // Need to delete here because of additional features of the derived class
+  // wrt colvarbias base class
+  delete_features();
 }