fixes file open

cpp/particles/interpolation/abstract_particle_set.hpp

@@ -290,7 +290,10 @@ class abstract_particle_set
             this->sample(*field_to_sample, pdata.get());
             hid_t file_id = -1;
             if (field_to_sample->myrank == 0)
-                hid_t file_id = H5Fopen(file_name.c_str(), H5F_ACC_RDWR, H5P_DEFAULT);
+            {
+                file_id = H5Fopen(file_name.c_str(), H5F_ACC_RDWR, H5P_DEFAULT);
+                assert(file_id > 0);
+            }
             hdf5_tools::gather_and_write_with_single_rank(
                     field_to_sample->myrank,
                     0,