ac3_fixed: fix out-of-bound read

Should also improve decoding, but actually doesn't...

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit b083440060)

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

libavcodec/ac3dec.c

@@ -872,7 +872,7 @@ static int decode_audio_block(AC3DecodeContext *s, int blk)
                 start_subband += start_subband - 7;
             end_subband    = get_bits(gbc, 3) + 5;
 #if USE_FIXED
-            s->spx_dst_end_freq = end_freq_inv_tab[end_subband];
+            s->spx_dst_end_freq = end_freq_inv_tab[end_subband-5];
 #endif
             if (end_subband   > 7)
                 end_subband   += end_subband   - 7;