avcodec_open2 segfault for h264_vaapi

[kurufu]

Summary of the bug:
avcodec_open2 segfaults if you try to open vaapi with well formed but incorrect options.

How to reproduce:

#include <libavcodec/avcodec.h>    
    
int main(int argc, char *argv[]) {    
  AVDictionary *opts = NULL;             
  av_dict_set(&opts, "b", "2.5M", 0);                      
  AVCodec *codec = avcodec_find_encoder_by_name("h264_vaapi");     
  if (!codec)                      
    exit(1);                     
                                              
  AVCodecContext *context = avcodec_alloc_context3(codec);    
  context->time_base = (AVRational){1, 60};       
  context->pix_fmt = AV_PIX_FMT_VAAPI;             
  context->width = 640;                            
  context->height = 480;                           
                                                   
  if (avcodec_open2(context, codec, &opts) < 0)    
    exit(1);                                
  exit(0);    
} 

Build with gcc -o example main.c -lavcodec -lavutil on ffmpeg 4.4.1. Though the given options are not enough to correctly open the codec, (the error [h264_vaapi @ 0x55fc49a13c00] A hardware frames reference is required to associate the encoding device. is reported). Libavcodec then proceeds to segfault.

Preferably ffmpeg would not segfault when provided well formed input. Marking important as its a crash, feel free to re-prioritize if thats not the correct choice. Also preferably a fix would be backported if possible.

[mkver]

Fixed in d1b47f3bfcc625ca1cae210fc198dcbd54381a88.