avcodec/vorbis: fix decoding of single element huffman trees

Fixes Ticket2893
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/vorbis.c

@@ -85,6 +85,11 @@ int ff_vorbis_len2vlc(uint8_t *bits, uint32_t *codes, unsigned num)
 
     ++p;
 
+    for (i = p; (bits[i] == 0) && (i < num); ++i)
+        ;
+    if (i == num)
+        return 0;
+
     for (; p < num; ++p) {
         if (bits[p] > 32)
              return AVERROR_INVALIDDATA;