vulkan_h264: check if slices_buf exists on end_frame

The issue is that while decode_slice is guaranteed to never get
called without start_frame, end_frame is not. Moreover, it is
not guaranteed it won't be called twice.

On a badly-broken sample, this is what happens, which leads to
a segfault, as vp->slices_buf doesn't exist, as it has been handed
off for decoding already and isn't owned by the frame.

Return an error as it's indicative that it's a corrupt stream rather
than just missing any slices.

Prevents a segfault.

libavcodec/vulkan_h264.c

@@ -520,6 +520,9 @@ static int vk_h264_end_frame(AVCodecContext *avctx)
     if (!hp->h264_pic_info.sliceCount)
         return 0;
 
+    if (!vp->slices_buf)
+        return AVERROR(EINVAL);
+
     if (!dec->session_params) {
         int err = vk_h264_create_params(avctx, &dec->session_params);
         if (err < 0)