MP3File: Avoid crash in get_buffer when deinitted

When a playing mp3 is deinitted, it's possible to reach get_buffer,
but all the internal pointers are NULL.  This would lead to a hard fault.
Avoid it by returning GET_BUFFER_ERROR instead.

shared-module/audiomp3/MP3File.c

@@ -227,6 +227,9 @@ audioio_get_buffer_result_t audiomp3_mp3file_get_buffer(audiomp3_mp3file_obj_t*
                                                         uint8_t channel,
                                                         uint8_t** bufptr,
                                                         uint32_t* buffer_length) {
+    if (!self->inbuf) {
+        return GET_BUFFER_ERROR;
+    }
     if (!single_channel) {
         channel = 0;
     }