LIEF fails to analyze some MS-DOS executable

[Wenzel]

With #101 's PR, we use LIEF to analyze MS-DOS executables.
Some of them return None after the LIEF parsing.

@milenkowski here is the archive containing all the failed executables: (windows XP)
ec8e6f82-5d94-4cfd-984f-f5a5ed713dfc_static_analyzer_failed.zip