audio_route: fix possible segfault

* Mixers may not allocate memory to save their values if the type is not
  of bool, enum, or int. However we do not check the type when actually
  attempting to use the allocated memory, resulting in possibly writing
  into random memory.

Change-Id: I8e59c8e6211b7ea10bd128069378632e0ced27a5

Conflicts:
	audio_route/audio_route.c