avcodec: fix double free on error

(cherry picked from commit 9bf41d6aeb35a121ce9c96adb21852ca69eb106b)

modules/codec/avcodec/audio.c

@@ -361,6 +361,7 @@ block_t * DecodeAudio ( decoder_t *p_dec, block_t **pp_block )
     if( p_block->i_buffer == 0 )
     {   /* Done with this buffer */
         block_Release( p_block );
+        p_block = NULL;
         *pp_block = NULL;
     }
 
@@ -448,7 +449,8 @@ block_t * DecodeAudio ( decoder_t *p_dec, block_t **pp_block )
 end:
     *pp_block = NULL;
 drop:
-    block_Release(p_block);
+    if( p_block != NULL )
+        block_Release(p_block);
     return NULL;
 }