testcrypt: refactor return_opt_foo functions.

There are already three tediously similar functions that wrap a NULL
check around some existing function to return one or another kind of
pointer, and I'm about to want to add another one. Make a macro so
that it's easy to make more functions identical to these three.

testcrypt.c

@@ -503,29 +503,18 @@ static void return_val_string_asciz(strbuf *out, char *s)
     return_val_string(out, sb);
 }
 
-static void return_opt_val_string_asciz(strbuf *out, char *s)
+#define NULLABLE_RETURN_WRAPPER(type_name, c_type)                      \
-{
+    static void return_opt_##type_name(strbuf *out, c_type ptr)         \
-    if (!s)
+    {                                                                   \
-        strbuf_catf(out, "NULL\n");
+        if (!ptr)                                                       \
-    else
+            strbuf_catf(out, "NULL\n");                                 \
-        return_val_string_asciz(out, s);
+        else                                                            \
-}
+            return_##type_name(out, ptr);                               \
+    }
 
-static void return_opt_val_cipher(strbuf *out, ssh_cipher *c)
+NULLABLE_RETURN_WRAPPER(val_string_asciz, char *)
-{
+NULLABLE_RETURN_WRAPPER(val_cipher, ssh_cipher *)
-    if (!c)
+NULLABLE_RETURN_WRAPPER(val_hash, ssh_hash *)
-        strbuf_catf(out, "NULL\n");
-    else
-        return_val_cipher(out, c);
-}
-
-static void return_opt_val_hash(strbuf *out, ssh_hash *h)
-{
-    if (!h)
-        strbuf_catf(out, "NULL\n");
-    else
-        return_val_hash(out, h);
-}
 
 static void handle_hello(BinarySource *in, strbuf *out)
 {