vp3: avoid buffer overread in coeff decode

I couldn't measure it to be slower for normal interframe videos.
For the worst case, high-bitrate intra-only videos, it can be 0.7% slower.

Originally committed as revision 22416 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/vp3.c

@@ -941,7 +941,7 @@ static int unpack_vlcs(Vp3DecodeContext *s, GetBitContext *gb,
     if (blocks_ended)
         dct_tokens[j++] = blocks_ended << 2;
 
-    while (coeff_i < num_coeffs) {
+    while (coeff_i < num_coeffs && get_bits_left(gb) > 0) {
             /* decode a VLC into a token */
             token = get_vlc2(gb, vlc_table, 5, 3);
             /* use the token to get a zero run, a coefficient, and an eob run */