avformat/avidec: initialize header array

Fixes use of uninitialized memory in case read fails
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/avidec.c

@@ -136,7 +136,7 @@ static inline int get_duration(AVIStream *ast, int len)
 static int get_riff(AVFormatContext *s, AVIOContext *pb)
 {
     AVIContext *avi = s->priv_data;
-    char header[8];
+    char header[8] = {0};
     int i;
 
     /* check RIFF header */