Try harder when looking for crash thread

The current algorithm cannot handle backtraces with multiple threads
containing the designated crash frame such as [1]. We just pick the
first one (from sorted threads) with the risk that our guess might be
wrong. Fixes #3.

[1] https://bugzilla.redhat.com/attachment.cgi?id=703269

Signed-off-by: Martin Milata <mmilata@redhat.com>
Signed-off-by: Jakub Filak <jfilak@redhat.com>