avcodec/nvdec: Increase frame pool size to help deinterlacing

With the cuda yadif filter in use, the number of mapped decoder
frames could increase by two, as the filter holds on to additional
frames.

(cherry picked from commit 1b41115ef7)

libavcodec/nvdec.c

@@ -601,7 +601,11 @@ int ff_nvdec_frame_params(AVCodecContext *avctx,
     frames_ctx->format            = AV_PIX_FMT_CUDA;
     frames_ctx->width             = (avctx->coded_width + 1) & ~1;
     frames_ctx->height            = (avctx->coded_height + 1) & ~1;
-    frames_ctx->initial_pool_size = dpb_size;
+    /*
+     * We add two extra frames to the pool to account for deinterlacing filters
+     * holding onto their frames.
+     */
+    frames_ctx->initial_pool_size = dpb_size + 2;
 
     frames_ctx->free = nvdec_free_dummy;
     frames_ctx->pool = av_buffer_pool_init(0, nvdec_alloc_dummy);