Fix import of single-frame Matroska files
Fix import of single-frame Matroska files

It seems possible that multiple frames could be lost here, but I don't know
what kind of file would trigger it. Added an error log.

Fixes #558


git-svn-id: d8f21eb7283bfb39ebab2f8f9b4a259b233f9348@1399 621663c8-3916-0410-8f58-edc14a8543d5

mrvacbob/perian,mrvacbob/perian,mrvacbob/perian,mrvacbob/perian,mrvacbob/perian