avoid segfaults if a dump is created and followed by run pre no

src/dump_grid.cpp

@@ -311,6 +311,8 @@ void DumpGrid::init_style()
 
 void DumpGrid::write_header(bigint ndump)
 {
+  if (!header_choice) error->all(FLERR, "Must not use 'run pre no' after creating a new dump");
+
   if (multiproc) (this->*header_choice)(ndump);
   else if (me == 0) (this->*header_choice)(ndump);
 }