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/global.h

@@ -102,6 +102,9 @@ void *snd_dlsym(void *handle, const char *name, const char *version);
 int snd_dlclose(void *handle);
 
 
+/** \brief alloca helper macro. */
+#define __snd_alloca(ptr,type) do { *ptr = (type##_t *) alloca(type##_sizeof()); memset(*ptr, 0, type##_sizeof()); } while (0)
+
 /**
  * \brief Internal structure for an async notification client handler.
  *