avcodec/vorbis: Fix another 1 << 31 > int32_t::max() with 1u.

Didn't notice this one when 9648cc6d was landed.
Signed-off-by: Dale Curtis <dalecurtis@chromium.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/vorbis.c

@@ -67,7 +67,7 @@ int ff_vorbis_len2vlc(uint8_t *bits, uint32_t *codes, unsigned num)
     if (bits[p] > 32)
         return AVERROR_INVALIDDATA;
     for (i = 0; i < bits[p]; ++i)
-        exit_at_level[i+1] = 1 << i;
+        exit_at_level[i+1] = 1u << i;
 
     ++p;