ac3: Check the array bound before dereferencing

CC: libav-stable@libav.org
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavcodec/ac3enc_template.c

@@ -260,7 +260,7 @@ static void apply_channel_coupling(AC3EncodeContext *s)
                 energy_cpl = energy[blk][CPL_CH][bnd];
                 energy_ch = energy[blk][ch][bnd];
                 blk1 = blk+1;
-                while (!s->blocks[blk1].new_cpl_coords[ch] && blk1 < s->num_blocks) {
+                while (blk1 < s->num_blocks && !s->blocks[blk1].new_cpl_coords[ch]) {
                     if (s->blocks[blk1].cpl_in_use) {
                         energy_cpl += energy[blk1][CPL_CH][bnd];
                         energy_ch += energy[blk1][ch][bnd];