audiomixer: The pad's size is always supposed to be the whole buffer size

And the offset the offset into that buffer. Changing the size will
cause all kinds of assumptions to fail and cause crashes.

gst/audiomixer/gstaudiomixer.c

@@ -1114,10 +1114,9 @@ gst_audio_mixer_fill_buffer (GstAudioMixer * audiomixer, GstAudioMixerPad * pad,
     if (start_running_time_offset < audiomixer->offset) {
       GstBuffer *buf;
       guint diff = (audiomixer->offset - start_running_time_offset) * bpf;
+
       pad->position += diff;
-      pad->size -= diff;
-      /* FIXME: This could only happen due to rounding errors */
-      if (pad->size == 0) {
+      if (pad->position >= pad->size) {
         /* Empty buffer, drop */
         gst_buffer_unref (inbuf);
         pad->buffer = NULL;