vaapi_encode: Add an assert in vaapi_encode_truncate_gop()

The flag of input_available must be set when pic_start is not NULL, so
add an assert to ensure it is true. In addition, the assert on last_pic
is unnecessary now, so remove this assert.
Signed-off-by: Haihao Xiang <haihao.xiang@intel.com>

libavcodec/vaapi_encode.c

@@ -762,6 +762,8 @@ static int vaapi_encode_truncate_gop(AVCodecContext *avctx)
     VAAPIEncodeContext *ctx = avctx->priv_data;
     VAAPIEncodePicture *pic, *last_pic, *next;
 
+    av_assert0(!ctx->pic_start || ctx->pic_start->input_available);
+
     // Find the last picture we actually have input for.
     for (pic = ctx->pic_start; pic; pic = pic->next) {
         if (!pic->input_available)
@@ -770,8 +772,6 @@ static int vaapi_encode_truncate_gop(AVCodecContext *avctx)
     }
 
     if (pic) {
-        av_assert0(last_pic);
-
         if (last_pic->type == PICTURE_TYPE_B) {
             // Some fixing up is required.  Change the type of this
             // picture to P, then modify preceding B references which