Fix MP3 heuristic check:

- for short files that prematurely result in status 'MPG123_DONE', no erroneous reads should be allowed
- for some files, mpg123_read might immediately return status 'MPG123_DONE' without actually reading anything -> add a check for output param 'done'

Author: florianessl

Diff for: src/decoder_mpg123.cpp

@@ -230,8 +230,8 @@ bool Mpg123Decoder::IsMp3(Filesystem_Stream::InputStream& stream) {
 	// Read beginning of assumed MP3 file and count errors as an heuristic to detect MP3
 	for (int i = 0; i < 10; ++i) {
 		err = mpg123_read(decoder.handle.get(), buffer, 1024, &done);
-		if (err == MPG123_DONE) {
-			return true;
+		if (err == MPG123_DONE && done != 0) {
+			return err_count == 0;
 		}
 		if (err != MPG123_OK) {
 			err_count += 1;