Eliminate a correct but harmless 'unititialized variable' warning from libvorbisfile compile.

svn path=/trunk/vorbis/; revision=16246

lib/vorbisfile.c

@@ -229,8 +229,8 @@ static ogg_int64_t _get_prev_page_serial(OggVorbis_File *vf,
 
   ogg_int64_t prefoffset=-1;
   ogg_int64_t offset=-1;
-  ogg_int64_t ret_serialno;
-  ogg_int64_t ret_gran;
+  ogg_int64_t ret_serialno=-1;
+  ogg_int64_t ret_gran=-1;
 
   while(offset==-1){
     begin-=CHUNKSIZE;