avcodec/vaapi_encode: Check hwctx

Fixes: null pointer dereference
Fixes: 70376/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_H264_VAAPI_fuzzer-4733551250046976

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/vaapi_encode.c

@@ -2307,11 +2307,13 @@ av_cold int ff_vaapi_encode_close(AVCodecContext *avctx)
     ff_refstruct_pool_uninit(&ctx->output_buffer_pool);
 
     if (ctx->va_context != VA_INVALID_ID) {
+        if (ctx->hwctx)
             vaDestroyContext(ctx->hwctx->display, ctx->va_context);
         ctx->va_context = VA_INVALID_ID;
     }
 
     if (ctx->va_config != VA_INVALID_ID) {
+        if (ctx->hwctx)
             vaDestroyConfig(ctx->hwctx->display, ctx->va_config);
         ctx->va_config = VA_INVALID_ID;
     }