nutdec: check that filesize is valid before using it.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/nutdec.c

@@ -562,6 +562,9 @@ static int find_and_decode_index(NUTContext *nut)
     int8_t *has_keyframe;
     int ret = -1;
 
+    if(filesize <= 0)
+        return -1;
+
     avio_seek(bc, filesize - 12, SEEK_SET);
     avio_seek(bc, filesize - avio_rb64(bc), SEEK_SET);
     if (avio_rb64(bc) != INDEX_STARTCODE) {