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.

include/asoundlib-head.h

@@ -33,7 +33,6 @@
 #include <stdlib.h>
 #include <string.h>
 #include <fcntl.h>
-#include <assert.h>
 #include <endian.h>
 #include <sys/poll.h>
 #include <errno.h>