vulkan_h264: reject end_frame being called without valid session parameters

When seeking through MBAFF-coded H264, this can happen. Decoding calls end_frame
without calling start_frame. We are unable to decode this, as no frame
state has been set.

Happens for both VAAPI and Vulkan. Could be an issue elsewhere, hence
the individual commit.

libavcodec/vulkan_h264.c

@@ -508,10 +508,14 @@ static int vk_h264_end_frame(AVCodecContext *avctx)
     const H264Context *h = avctx->priv_data;
     H264Picture *pic = h->cur_pic_ptr;
     H264VulkanDecodePicture *hp = pic->hwaccel_picture_private;
+    FFVulkanDecodeContext *dec = avctx->internal->hwaccel_priv_data;
     FFVulkanDecodePicture *vp = &hp->vp;
     FFVulkanDecodePicture *rvp[H264_MAX_PICTURE_COUNT] = { 0 };
     AVFrame *rav[H264_MAX_PICTURE_COUNT] = { 0 };
 
+    if (!dec->session_params)
+        return AVERROR(EINVAL);
+
     for (int i = 0; i < vp->decode_info.referenceSlotCount; i++) {
         H264Picture *rp = hp->ref_src[i];
         H264VulkanDecodePicture *rhp = rp->hwaccel_picture_private;