Fix crash in mincconcat when header cannot be expanded

This patch fixes a segmentation fault that occurs when an input file has a filename extension that indicates compression (e.g. ".bz2"), but doesn't really contain content in the corresponding compression format. In this case exit with an error message instead of just crashing.
Merge pull request #15 from martin-steghoefer/fix-mincconcat-crash

Fix crash in mincconcat when header cannot be expanded