Fix Theora decoding bug if loopfilter is completely disabled

Bug caused by the fact that get_bits(gb, 0) is undefined.
Doesn't affect any streams generated by the official Theora encoder, but such
streams are nevertheless valid.

Fixes decoding of CELT-933dd833-nmr-bandt.ogv.

Originally committed as revision 25573 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/vp3.c

@@ -2052,6 +2052,7 @@ static int theora_decode_tables(AVCodecContext *avctx, GetBitContext *gb)
     if (s->theora >= 0x030200) {
         n = get_bits(gb, 3);
         /* loop filter limit values table */
+        if (n) {
         for (i = 0; i < 64; i++) {
             s->filter_limit_values[i] = get_bits(gb, n);
             if (s->filter_limit_values[i] > 127) {
@@ -2059,6 +2060,7 @@ static int theora_decode_tables(AVCodecContext *avctx, GetBitContext *gb)
                 s->filter_limit_values[i] = 127;
             }
         }
+        }
     }
 
     if (s->theora >= 0x030200)