fftools/ffmpeg: return an error from assert_avoptions() instead of aborting

Rename it to check_avoptions().

fftools/ffmpeg.c

@@ -457,13 +457,15 @@ void remove_avoptions(AVDictionary **a, AVDictionary *b)
     }
 }
 
-void assert_avoptions(AVDictionary *m)
+int check_avoptions(AVDictionary *m)
 {
     const AVDictionaryEntry *t;
     if ((t = av_dict_get(m, "", NULL, AV_DICT_IGNORE_SUFFIX))) {
         av_log(NULL, AV_LOG_FATAL, "Option %s not found.\n", t->key);
-        exit_program(1);
+        return AVERROR_OPTION_NOT_FOUND;
     }
+
+    return 0;
 }
 
 void update_benchmark(const char *fmt, ...)