ff_ac3_apply_rematrixing: set flags to NULL

This makes the code more robust

Related to CID703820
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/ac3enc.c

@@ -273,7 +273,7 @@ void ff_ac3_apply_rematrixing(AC3EncodeContext *s)
     int nb_coefs;
     int blk, bnd, i;
     int start, end;
-    uint8_t *flags;
+    uint8_t *flags = NULL;
 
     if (!s->rematrixing_enabled)
         return;