Fix crash when input file cannot be expanded

This commit fixes a segmentation fault that occurs when an input file indicates compression (e.g. by having an extension ".bz2", ".gz", etc.), 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 #19 from martin-steghoefer/fix-mincaverage-crash

Fix crash when input file cannot be expanded