Bug in ArchiveReader

In querying a msgpack archive for a entry that is not part of the archive, the code goes an infinite loop ca. line 363 in archive.py.

In the latest v0.8.0 commit (01.03), I included a test springer msgpack database which can be used to verify this behavior. For example one can do

from nomad.archive import query_archive

q = query_archive('springer.msg', {'281':'*'})

Spacegroup '281' of course is not part of the archive.