Mozilla #499604, annodex #486: avoid uninitialized variable,

bail out of oggz_seek_end() immediatly if get_prev_start_page() fails.

src/liboggz/oggz_seek.c

@@ -795,13 +795,13 @@ oggz_seek_end (OGGZ * oggz, ogg_int64_t unit_offset)
 
   offset_end = oggz_get_prev_start_page (oggz, og, &granulepos, &serialno);
 
-  unit_end = oggz_get_unit (oggz, serialno, granulepos);
-
   if (offset_end < 0) {
     oggz_reset (oggz, offset_orig, -1, SEEK_SET);
     return -1;
   }
 
+  unit_end = oggz_get_unit (oggz, serialno, granulepos);
+
 #ifdef DEBUG
   printf ("*** oggz_seek_end: found packet (%lld) at @%" PRI_OGGZ_OFF_T "d [%lld]\n",
 	  unit_end, offset_end, granulepos);