Fix: Heap Buffer Overflow

[ax3l]

Fix a memory violation when run in serial.

Found via gcc's -fsanitize=address sanitizer.

[johnbent]

Excellent. But I think it’d be slightly cleaner if line 264 was simply ‘ret = 1’. In fact, if num_ranks is 1, there is no need for any of the MPI communications in this function.

[ax3l]

Oh yes, fully agree. In fact, I was a bit confused why it reduces to index 1 anyway.
I cannot simply return there, to avoid leaks, but we could return earlier.

[ax3l]

Not sure if I complicate the source further with early returns since also some logs are written here. Probably it's clearer already with the last push?

[JulianKunkel]

I think this is alright, given the simplicity... Good catch.