avcodec: handle NULL pp_block

Drain when pp_block is NULL.

modules/codec/avcodec/video.c

@@ -530,9 +530,6 @@ static picture_t *DecodeVideo( decoder_t *p_dec, block_t **pp_block )
     int b_drawpicture;
     block_t *p_block;
 
-    if( !pp_block )
-        return NULL;
-
     if( !p_context->extradata_size && p_dec->fmt_in.i_extra )
     {
         ffmpeg_InitCodec( p_dec );
@@ -540,7 +537,7 @@ static picture_t *DecodeVideo( decoder_t *p_dec, block_t **pp_block )
             OpenVideoCodec( p_dec );
     }
 
-    p_block = *pp_block;
+    p_block = pp_block ? *pp_block : NULL;
     if(!p_block && !(p_sys->p_codec->capabilities & CODEC_CAP_DELAY) )
         return NULL;