Fix warning in case no streams are decodable.

Previously it assumed that if no streams are decodable, it is
due to an error. This can also happen without an error when
we simply don't recognize any of the codecs in use.

For example if someone uploaded an Ogg file using uncompressed
PCM audio, and dirac video.

Change-Id: I7f90725900af8c93a9edb35796cef895f51502f3
Merge "Fix warning in case no streams are decodable."