Do not call lseek() with invalid whence value

Originally committed as revision 21795 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/file.c

@@ -73,6 +73,8 @@ static int file_write(URLContext *h, unsigned char *buf, int size)
 static int64_t file_seek(URLContext *h, int64_t pos, int whence)
 {
     int fd = (intptr_t) h->priv_data;
+    if (whence != SEEK_SET && whence != SEEK_CUR && whence != SEEK_END)
+        return AVERROR_NOTSUPP;
     return lseek(fd, pos, whence);
 }