OutputString should include null terminator

The allocation of OutputString in vtkDataWriter needs to be
size()+1 to leave room for the null terminator.

Change-Id: I96af49cc7307f1e0cefd2edc3be8617edfc3a8e3
Merge topic 'fix-datawriter-memory-corruption' into master

f3f02141 OutputString should include null terminator