MP3Decoder: better handle underflows of mp3 stream

 * Don't consume in the case of "indata overflow".
   Doing so leaves us at a bad boundary within the MP3 data
   and can continue decoding from an inappropriate spot
   i.e., one that looks likede mp3 data but is NOT.

   because there are many crashing bugs in the helix mp3 library on
   invalid input data, we must do our best to avoid them, and this
   is one way to do that.

 * clear the output buffer in the case there's not a sync word in the
   buffer. this can also happen when too little data is available.

   this changes more "stuttering" conditions into "silent" conditions.

With these changes, I can get through 3+ plays of "idea.mp3" from a local
http server with long pauses (but not stuttering glitches or safe mode
crashes).

I was also able to play through 10+ minutes of http://ice2.somafm.com/dronezone-128-mp3
without crashing or "end of stream", though again there are pauses
due to packet loss.

I think this is good now, except for the problems that arise when
the socket layer doesn't deliver a fresh packet for a long time.

Author: jepler

shared-module/audiomp3/MP3Decoder.c

@@ -447,14 +447,16 @@ audioio_get_buffer_result_t audiomp3_mp3file_get_buffer(audiomp3_mp3file_obj_t *
 
     mp3file_skip_id3v2(self, false);
     if (!mp3file_find_sync_word(self, false)) {
+        memset(buffer, 0, self->frame_buffer_size);
         *buffer_length = 0;
         return self->eof ? GET_BUFFER_DONE : GET_BUFFER_ERROR;
     }
     int bytes_left = BYTES_LEFT(self);
     uint8_t *inbuf = READ_PTR(self);
     int err = MP3Decode(self->decoder, &inbuf, &bytes_left, buffer, 0);
-    CONSUME(self, BYTES_LEFT(self) - bytes_left);
-
+    if (err != ERR_MP3_INDATA_UNDERFLOW) {
+        CONSUME(self, BYTES_LEFT(self) - bytes_left);
+    }
     if (err) {
         memset(buffer, 0, frame_buffer_size_bytes);
         if (DO_DEBUG) {