dca: fix signed overflow in shift

Signed-off-by: Mans Rullgard <mans@mansr.com>

libavcodec/dca.c

@@ -909,7 +909,8 @@ static void qmf_32_subbands(DCAContext * s, int chans,
     for (subindex = 0; subindex < 8; subindex++) {
         /* Load in one sample from each subband and clear inactive subbands */
         for (i = 0; i < sb_act; i++){
-            uint32_t v = AV_RN32A(&samples_in[i][subindex]) ^ ((i-1)&2)<<30;
+            unsigned sign = (i - 1) & 2;
+            uint32_t v = AV_RN32A(&samples_in[i][subindex]) ^ sign << 30;
             AV_WN32A(&s->raXin[i], v);
         }