Reset file position

The file handle might be at EOF if the input .opus file is small. So
reset the file position properly before any further handling.

Fixes #78.

src/audio-in.c

@@ -145,6 +145,7 @@ input_format *open_audio_file(FILE *in, oe_enc_opt *opt)
 
         if (formats[j].id_func(buf, buf_filled))
         {
+            rewind(in);
             /* ok, we now have something that can handle the file */
             if (formats[j].open_func(in, opt, buf, buf_filled)) {
                 free(buf);