Remove assert from header files

Putting assert in the public macros isn't good idea at all.
Let's get rid of them.

Also, clean up snd*_alloca() functions to use a helper macro
instead of copy and paste.

src/mixer/simple_none.c

@@ -35,6 +35,7 @@
 #include <string.h>
 #include <fcntl.h>
 #include <sys/ioctl.h>
+#include <assert.h>
 #include <math.h>
 #include <alsa/asoundlib.h>
 #include "mixer_simple.h"