lavfi/src_movie: Check pointer is not NULL before deref

Also do not check against empty string, the lower levels should
be able to deal with it.
Signed-off-by: Alexander Strasser <eclipse7@gmx.net>

libavfilter/src_movie.c

@@ -197,7 +197,7 @@ static av_cold int movie_common_init(AVFilterContext *ctx)
     char name[16];
     AVStream *st;
 
-    if (!*movie->file_name) {
+    if (!movie->file_name) {
         av_log(ctx, AV_LOG_ERROR, "No filename provided!\n");
         return AVERROR(EINVAL);
     }