* Also do default initialization with memset, this ensures a file is not
  opened and immediately closed by one collector.

* This hopefully fixes a problem introduced with support for
  differential timesteps.

* The previous coding assumed that some non-zero amount of data
  would be written to every file opened.

* This is needed in preparation to explicitly handle streamDefTimestep in
  the MPI parallelized version of the library.

* Fixes #1207