[FIX] Bugfix for threading issue in MascotGenericFile

- Fixes an error introduced by multiple threads: under certain
  conditions the global variable has_next could already be set to false
  before it is checked by a thread that still has work to do on a last
  spectrum. In this case the last spectra would not get added to the
  MSExperiment container.

Fixes #683
Merge pull request #684 from hroest/fix/MascotGenericFile

[FIX] Bugfix for threading issue in MascotGenericFile