zero-fill the grid

nifty_gridder.cc

@@ -1267,6 +1267,8 @@ template<typename T> pyarr_c<complex<T>> apply_holo(
   auto ogrid = res.mutable_data();
   {
   py::gil_scoped_release release;
+
+  for (size_t i=0; i<nu*nv; ++i) ogrid[i] = T(0);
   T beta = gconf.Beta();
   size_t w = gconf.W();
 
@@ -1307,7 +1309,7 @@ template<typename T> pyarr_c<complex<T>> apply_holo(
       complex<T> tmp(r*ku[cu]);
       for (size_t cv=0; cv<w; ++cv)
         wptr[cv] += tmp*kv[cv];
-      wptr+=jump;
+      wptr += jump;
       }
     }
 }