avformat/vapoursynth: properly initialize err variable in read_header_vs()

The variable "err" is not initialized, and set only when something went wrong.
When everything is OK, nobody sets it, so using it result in UB.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/vapoursynth.c

@@ -177,7 +177,7 @@ static av_cold int read_header_vs(AVFormatContext *s)
     char dummy;
     const VSVideoInfo *info;
     struct VSState *vss_state;
-    int err;
+    int err = 0;
 
     vss_state = av_mallocz(sizeof(*vss_state));
     if (!vss_state) {