Fix822 ffp early stop with open mp

[NikEfth]

This is a small patch to fix early stop when forward projecting with OpenMP.
The issue is not 100% reproducible but I have experienced many times.

[NikEfth]

@KrisThielemans Please wait before you review this. It will fail without OpenMP, I forgot to fix that.

[KrisThielemans]

I don't think we've ever seen this problem, @AnderBiguri and @danieldeidda can probably comment as they both have "monster" machines. But obviously that doesn't mean that it doesn't exist (wtih multi-threading, you never know).

However, I cannot see why this modification would help. In the original code, the viewgrams are a local variable, so they are thread-specific already.

[NikEfth]

I agree, that viewgrams is local and there should not be a difference.
As you know for the normalisation code I had to forward project a model of the source used in the simulation.
Many times, the projection was finishing without reported problems, but a few megabytes short.
With the suggested modification I have not faced the problem again.

[NikEfth]

P.S.
Maybe is an OPENMP version, specific problem. I am using Arch Linux, which means I have the latest version.