Try harder when looking for crash thread

Backported from btparser 6a1db1e193db4136f2b170b911a946c30001fd98:

  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 abrt/btparser#3.

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

Related to rhbz#994747.

Signed-off-by: Martin Milata <mmilata@redhat.com>
Signed-off-by: Jakub Filak <jfilak@redhat.com>
Always choose a crash thread

If we don't know the crash thread, just choose the first one in order to
generate a valid uReport. Closes rhbz#994747.

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