Fix ASYNC null implementation

The ASYNC null implementation has not kept pace with the rest of the async
development and so was failing to compile.

Reviewed-by: Rich Salz <rsalz@openssl.org>

crypto/async/arch/async_null.h

@@ -73,4 +73,8 @@ typedef struct async_fibre_st {
 # define ASYNC_FIBRE_makecontext(c)
 # define ASYNC_FIBRE_free(f)
 # define ASYNC_FIBRE_init_dispatcher(f)
+# define async_pipe(f)                          0
+# define async_write1(f,b)                      ((*b == 0) ? (f = 0) : (f = 1))
+# define async_read1(f,b)                       (*b = 0)
+
 #endif