This is a followup commit to b95725c3

* Resetting the decoder should only happen when we get a new initialization
header (0x01) and not on the other headers
* The initialized variable only gets set to TRUE once all headers have
been parsed. Also check if the vorbis_info struct has been properly resetted
also. Failure to do that would cause vorbisdec to error if it got
two initialization header in a row (the first would configure the underlying
library and the second one would error out because it's already initialized)

https://bugzilla.gnome.org/show_bug.cgi?id=779515